ZooCVABioIII.2013.Abante

Comparative Anatomy
of Vertebrates
Presented by: Daylo A. Abante
BS-Biology III
By: Editha De Jesus

Comparative Vertebrate Anatomy
• Study of vertebrate structure (or morphology)
from an evolutionary perspective and the
functional aspects of these structures.
– As a result, an understanding of phylogenesis is
essential (i.e.; an understanding of the ancestral
history of the vertebrate orders).
– Evolutionary developments and relationships can
be gathered by looking at embryological, and to a
lesser extent fetal, development. Ontogenesis is
also very important.

The Phylum Chordata
• The phylum chordata is made up of a group of
animals all possessing four common features.
– Notochord, a rigid cartilaginous rod defining the
longitudinal axis in the embryo
– Dorsal Hollow Nerve Cord, the spinal cord an brain
– Postanal Tail
– Endostyle, a glandular groove in the floor of the
pharynx.

The Phylum Chordata
• The phylum can be divided into three
subphyla.
– Urochordata, the tunicates and sea squirts
– Cephalochordata, amphioxus
– Vertebrata/Craniata, chordates having a vertebral
column of bone or cartilage

Regional Differentiation
• The typical vertebrate body has three regions
– head
– trunk
– Tail
• The development of the head is a developmental
process termed Cranialization.
– The head contains the brain, a number, of special
sense organs, often jaws, and gills in fishes for
respiration.
– The trunk contains the Coelom, a body cavity that
houses the visceral organs.

Regional Differentiation
• The coelom is surrounded by the body wall consisting of:
– muscles
– vertebrae
– ribs
• The trunks of the Gnathostomata , the jawed vertebrates
and most numerous vertebrates, will typically have paired
pelvic and pectoral appendages (ex; fins, wings, legs).
• Many vertebrates have a Neck. The neck is a narrow
structure connecting the head to the trunk.
– Necks are found in the tetrapods.
• The tail is postanal in vertebrates meaning that it originates
posterior to the anus. The tail is present in all embryonic
vertebrates but may be lacking in the adult form.

Bilateral Symmetry
• Bilateral symmetry means that the vertebrate
body can be divided into two equal right and
left hand portions.
• This allows vertebrate anatomy to be studied
by dividing the body into Planes.

Bilateral Symmetry
• Three major planes of the body:
– Transverse Plane (aka; horizontal plane, cross section-
aplane running horizontally (left to right) dividing the
body into inferior and superior portions.
– Sagittal Plane - a vertical plane dividing the body into
left and right portions.
• Midsagittal (or Median Sagittal) Plane - runs along the
midline of the body.
• Parasagittal Plane - runs at other than the midline of the
body.
– Frontal Plane (aka; Coronal Plane) - a vertical plane
which divides the body into anterior and posterior
portions.

Segmentation or Metamerism
• It is the serial repetition of structures along
the long axis of the body.
• It is evident in the embryo.
• It is also present in the adult body to varying
degrees.
– Examples: muscle segments of fishes, rectus
abdominis in humans, ribs

Notochord and Vertebral Column
• The notochord is a cartilaginous rod that
defines the long axis of the embryo.
– the first skeletal feature to appear in the embryo.
– located immediately ventral to the developing
nerve cord and superior to the digestive tube.
• During development, in most vertebrates,
bony or cartilaginous vertebrae will grow
around the notochord

• The weight bearing portion of the vertebra,
the centrum , will surround the notochord.
– As to how much of the notochord will be retained
in the adult animal is variable.
• Most fishes maintain the notochord in their adult form.
• Some amphibians will show a pattern similar to that
seen in fishes.
• In reptiles, birds, and mammals the notochord is
almost or completely lost.
– In mammals remnants of the notochord will remain as
portions of the intervertebral discs (called the Nucleus
Pulposus).
– In modern reptiles and birds even this vestige has been lost.

• In Agnathans (jawless vertebrates) the
notochord grows and is maintained
throughout life. It will even develop
extensions called Lateral Neural Cartilages
that will be found lateral to the spinal cord.

• The Vertebral or Neural Arch will extend
dorsally from the centra to surround and
protect the spinal cord.
• A number of processes will radiate from the
arch.
• In some vertebrates a second arch, the Hemal
Arch , will extend from the centrum ventrally
in caudal vertebrae. The hemal arches
surround and protect the caudal artery.

Dorsal Hollow Nerve Cord
• The vertebrate nerve cord is dorsally oriented and
hollow.
– It consists of the brain and spinal cord.
– The hollow center of the vertebrate nerve cord is termed
the Neurocoel.
• The neurocoel includes the central canal of the spinal cord.
• The nerve cord originates by the process of
Neuralation.
– Neuralation typically occurs along the longitudinal axis of
the embryo dorsal to the notochord.
• A groove, termed the Neural Groove , will form dorsal to the
notochord.
• The neural groove will sink into the embryonic body and close off
to form the Neural Tube.

• In Agnathans and Neopterygians (the gars,
bowfins, and teleosts) neuralation has
become slightly modified from the typical
vertebrate pattern.
– The neural groove does not form.
– Instead a Neural Keel forms. The neural keel is a
wedge shaped ectodermal structure dorsal to the
notochord. It will separate from the surface
ectoderm, form a cavity, and become a typical
nerve cord

• The nerve cord expands anteriorly to form the
brain.
• Cranial and spinal nerves will develop and
radiate out from the nerve cord.
– These nerves allow for communication between
the CNS and the rest of the body.
• This is the job of the PNS.

Dorsal Hollow Nerve Cord in
Amphioxus

Pharynx
• The pharynx is a vital portion of the vertebrate
embryo.
– It shows the relationship between vertebrates and
other chordates.
– It produces a number of structures:
• gills in fish
• lungs in tetrapods
• jaw skeleton and musculature
• some endocrine glands
• the middle ear in tetrapods
• and serves a source of stem immune cells in the human
fetus.
– As a result, all vertebrate embryos will show a basic
pharyngeal architecture.

Pharynx: Development of Pharyngeal
Pouches and Pharyngeal Slits
• Pharyngeal pouches arise as outpocketings of gut
endoderm.
– These endodermal outpocketings will migrate towards
the surface.
• At the same time grooves in the above lying
ectoderm, termed Ectodermal Grooves, will grow
towards the developing pouches.
• Eventually the two structures will grow close
together and will be separated only by a thin
membrane of tissue called the Branchial Plate.

• If the branchial plate ruptures, a passageway forms
termed the Pharyngeal Slit.
– In fishes pharyngeal slits are maintained throughout life
for gills.
– In tetrapods pharyngeal slits are only temporary
structures.
• Examples:
– Frogs have 6 pharyngeal pouches in the embryonic form.
– Four will form the gill slits of the tadpole.
– These four slits will close up again during the
metamorphosis into the Adult frog

• Examples:
– Chick embryos will have six pouches. Pharyngeal
pouches number 1, 2, and 3 will rupture and then
close again.
– In mammals only one or two pharyngeal pouches will
rupture. The rupturing pouches tend to be anterior
pouches and will close up again.
• Pharyngeal pouch number one becomes the Eustachian
Tube.
• Pharyngeal pouch number two will eventually house the
palatine tonsils.
• Several posterior pharyngeal pouches will give rise to certain
endocrine glands.

Pharynx: The Pharyngeal Arches
• Between adjacent pharyngeal pouches/slits
are columns of tissue called the Pharyngeal
Arches
• Each pharyngeal arch has four components or
the blastemas from which these components
will arise.

• Typically these four components are:
– Pharyngeal or Visceral Skeleton- supportive skeletal
elements
– Branchniomeric Musculature- skeletal muscles to
operate the arch due to its relationship with the gills.
– Cranial nerve branches to innervate the arch
• These cranial nerve branches will be both sensory and motor
in function.
• The cranial nerves that send branches into the pharyngeal
arches are CN 5, 7, 9, & 10.
– Aortic arch to connect the dorsal and ventral aorta.

• The four components will also be found anterior
to the first pharyngeal arch and often posterior to
the last pharyngeal arch.
• The arches numbered from anterior to posterior.
– The anterior most arch is the First Pharyngeal Arch.
• The first pharyngeal arch is typically referred to as the
Mandibular Arch since it contains the upper and lower jaws
and related structures.
– The second arch is referred to as the Hyoid Arch.
– The remaining arches are numbered 3 through 7.
• Sometimes arches 3 through 7 are referred to as Branchial
Arches 1 through 5 since they resemble unmodified gill
arches.

Tube-Within-a-Tube Body Plan
• Vertebrates, and many invertebrates for that
matter, have a tube -within-a- tube body plan.
– The outer tube is the body wall.
– The inner tube is the digestive tract.
– The space between the outer and inner tubes,
between the body wall and digestive tact, is termed
the Coelom.
• The coelom in fishes, amphibians, and reptiles can be
divided into:
– The Pericardial Cavity which houses the heart
– The Pleuroperitoneal Cavity which houses the rest of the viscera.
» In tetrapods the lungs are located in this cavity.
– The pleuroperitoneal and pericardial cavities are separated by a
fibrous connective tissue partition termed the Transverse
Septum in tetrapods

Tube-Within-a-Tube Body Plan
• The coelom of birds and mammals is divided into:
– The Thoracic Cavity having Pericardial Cavity for the heart
and a pair of Pleural Cavity for the lungs.
– In some there will also be a Mediastinal Cavity
• Features that develop earliest in ontogeny are the
phylogenetically oldest features, having been inherited
from early common ancestors.
• Also features that develop later are of more recent
phylogenetic origin.
– An exception is the development of extraembryonic
membranes (ex; amnion) which develop later
phylogenetically but arise early ontogenetically due to the
survival needs of the embryo.

Other Vertebrate Body
Features

Integument
• The integument is composed of the skin and the
hypodermis.
– The skin is composed of the Epidermis and Dermis.
– The epidermis is a superficial layer composed of a
many layered epithelium having a number of
• functions.
• It produces a variety of glands serving a variety of purposes.
• It produces cornified appendages such as hair, feathers, and
scales.
• It provides protection from the external environment and
from pathogens.

Integument
• The dermis is deep to the epidermis and is a
connective tissue layer.
– In some animals it produces dermal bone.
– It will also aid in the production of scales.

Respiratory Structures
• Most vertebrates conduct external respiration
by means of extremely well vascularized
membranes.
• These membranes will be either located in the
pharyngeal pouches (i.e.; gills) or will be
derived from the pharyngeal floor (i.e.; lungs).

Digestive System
• The digestive system consists of a digestive tube and a
number of accessory organs.
– The Digestive Tube/Alimentary Canal/ Gastrointestinal Tract
is a long tube running from the mouth to the cloaca.
– It has a number of regional specializations along its length
but the entire tube shares the same basic histology.
– These specialized regions, or organs, include the oral cavity,
pharynx, esophagus, stomach, small intestine, and large
intestine.
– The Accessory Organs are located outside of the digestive
tube and release their products into the tube by means of
ducts. They include the liver, pancreas, and gall bladder.

Urogenital System
• The urinary and reproductive systems are often
considered as one combined system, the urogenital
system, due to their shared origins and structures.
– Examples: the kidneys and gonads both develop close
together on the coelomic roof, and both the male
reproductive and urinary systems utilize the urethra in
mammals.
• The urinary system removes wastes from the body so
as to maintainhomeostasis.
• The reproductive system allows for the propagation of
the species.

Circulatory System
• Vertebrates have a closed circulatory system
whereby blood is pumped by a muscular heart
through a series of blood vessels.

Skeletal System
• The vertebrate skeleton is internal (an
endoskeleton) and composed of cartilage/bone.
– It forms a framework providing for the shape of the
body, the anchoring of muscles for movement, and
protection of fragile organs.
• The skeleton can be divided into two portions:
– Axial Skeleton is composed of the skull, vertebrae,
and rib cage.
– Appendicular Skeleton is composed of the bones of
the limbs and their associated girdles.

Concepts, Premises,
and Pioneers
Chapter II

Homology
• Homology is similarity due to common ancestry.
– Initially all structures found in the same location with
the same appurtenance and function were assumed to
be homologous.
– Comparative Embryology has shown that not all look–
alike structures are homologous and some
homologues bear no resemblance to one another.
• Examples: the human stapes bone and the
hyomandibular cartilage of sharks,
• The intermaxillary bones of human embryos and
the premaxilla of adult apes.

Homology
• One way to test the veracity of suspected
homologues is to use comparative
embryology.
• If two structures (in two different animals)
come from the same embryonic precursor
then they are homologs.
• Another test is the structure’s location and the
evolutionary relationship of the two animals
in question.

Homoplasy
• The opposite of homology is homoplasy.
• Homoplasy is similarity due to any other cause
than common ancestry.
– Serial Homology is a term applied to segmental
structures that as a unit are homologous.
• Examples: spinal nerves of fishes and mammals.
• However, one segment of the entire unit may not
be homologous to the similar portion of a
metameric structure in another organism.
– Serial homology is concerned with the entire
metameric unit, not the individual components.

Analogy
• Analogy is the term for two structures that have similar
function but not necessarily the same origin. It is a
coincidental resemblance.
• Examples:
– The wing of a fly and a bird are analogous while the wing
of a bird and a bat are homologous.
– The horn of a bison and the antler of an elk are analogous.
• Sometimes homologous structures have a similar
function and are termed Analogous Homologues.
• Sometimes unrelated ,nonhomologous structures have
the same function and are termed Analogous
Homoplasies.

Wings of Fly vs. aves vs. mammal

Adaptation
• Adaptation is the ability of a species to
change due to some influence.
– Biological Adaptation is a hereditary modification
of a species structure, or phenotype, to increase
the probability of the species survival.
– Since phenotype is typically the result of
genotype, adaptations are believed to be a change
in the genetic make up of a species due to
environmental factors.

Adaptation
• Environmental factors result in genetic mutations
that, if beneficial, will be retained by the species.
• Preadaptation is the presence of traits that
enable a phenotype to handle new
environmental stresses prior to the to the
development of those stresses.
– Example: Lungs were present in fishes prior to the
movement of vertebrates onto the land. The lungs
served to allow some fish to utilize atmospheric
oxygen when dissolved oxygen in water was low.

Speciation
• Speciation is the development of two new species
from one original species.
– Often this involves the separation of one species into two
reproductively isolated populations.
– Over time, slight genetic differences will begin to
accumulate and the populations will begin to develop into
two genetically distinct species.
• This is termed Genetic Drift.
• Speciation is typically the result of geographical isolation coupled
with genetic change but other factors can play a role.
• Example:
– Polar bears evolved from brown bears.

Convergent Evolution
• Convergent Evolution is a form of evolution
where two or more unrelated organisms
acquire similar morphological features to exist
in similar environments.
– Example: sharks, ichthyosaurs, and dolphins; bats,
birds, and pterodactyls
– The two species can exist concurrently or be
separated by millions of years.
– Convergent evolution produces similar features in
two species that do not share a common ancestor.

Development
• Development is useful to in the study of Comparative
Vertebrate Anatomy because patterns of development
provide a further point of comparison among species.
– Comparative embryology is used to test the veracity of
homologies.
– Heterochrony is a change in the timing of developmental
events.
• Example: a delay in metamorphosis in some amphibians due to
environmental stresses.
• Heterochrony can result in speciation.
– One way is by causing the retention of juvenile
characteristics termed Paedomorphism.
• Example: the axolotyls

Ontogeny and Phylogeny
• Ontogeny/Ontogenesis is the developmental
history of an organism. It is the series of
morphological events that occur from zygote to
neonate, to adult, and even to death.
• Phylogeny is the evolutionary history of a group
or Taxon.
– Taxon is a related group of organisms
• Example: species, genus, family.
– Phylogeny chronicles the changes that occur over the
evolutionary history of a group while ontogeny
chronicles the changes that occur during the lifetime
of one individual.

• There is an old saying:” Ontogeny recapitulates
phylogeny.”
– This means that as you watch the development of an
individual, especially embryologically, you are also
watching the divergence of a species from its common
ancestors.
– However this is not quite true. Von BaerʼsLaw better
describes the relationship.
• Karl Ernst Von Baer was an embryologist who noted that the
features common to all members of a taxonomic group
develop earlier in ontogeny than do features that distinguish
subdivisions within the group.

• So features diagnostic of the subphylum appear earlier
in embryogenesis than do features that differentiate
families within the phylum.
• Today Von Baer’s Law has had a corollary added to it:
– Features that develop earliest in ontogeny are
thephylogeneticallyoldest features, having been inherited
from early common ancestors..
– Also features that develop later are of more recent
phylogenetic origin.
– An exception is the development of extraembryonic
membranes (ex; amnion) which develop later
phylogenetically but arise early ontogenetically due to the
survival needs of the embryo.

Definitions of Classification
• Systematics is the process by which organisms
are grouped in classification.
• Taxonomy is the conventions used to apply
names to these groups.
– Taxonomy is still based on the Linnaean system
established by Carolus Linnaeus; although it has
been refined.
– It uses a hierarchical system and binomial
nomenclature.

Taxonomy
• Hierarchical system: kingdom->phylum->class->order-
>family->genus->species
– Example of binomial classification; Canis lupus
• Currently systematics group organisms based on their
phylogenetic (or historical) relationships.
• In the past classification involved groupings based on
physical similarities but that can be misleading.
– Today Synamorphies are used.
• Synamorphies are shared derived characteristics.
– A phylogeny represents the current hypothesis as to the
relationship between species today.
• These hypotheses are open to testing.

Cladistics
• Cladistics is the methodology used by most systematics
today.
• Cladistics is based on the following assumptions:
– The hierarchy of relationships is knowledgeable and can be
represented in by branching pattern called a Cladogram.
• A cladogram is a useful tool to graphically display the
relationships between various organisms.
– The cladogram consists of a branching pattern with terminal
taxaat the end of each branch.
– Adjacent taxa represent sister taxa.
– Where the taxa split is the location of a common ancestor (often
this common ancestor is hypothetical).
– The branching pattern indicates relationships between taxa.

Cladistics
• Only the characteristics that distinguish a group are useful
in cladistics.
– Example: The presence of vertebra is not useful to distinguish
fish from birds.
• The distribution of derived characteristics on a branching
diagram determines homoplasy versus homology.
– Example: The distribution of derived characteristics on a
branching diagram determines the independent development of
similar structures versus the development of similar structures
due to common ancestry.
• The distribution of characteristics follows the rules of
parsimony:
– the solution that is most economical is usually correct.

Organic Evolution
and
Evolutionary Selection

Organic evolution
• Organic evolution is based on the concept that
organisms on earth have been changing and that those
species on earth today are descendants of earlier
species.
– This concept is partially based on fossil evidence.
• The Evolutionary Theory
– Jean Baptiste Lamarck(1744-1829) developed the concept
of the Doctrine of Acquired Characteristics.
• It states that structures which are used over successive generations
become stronger and better developed while structures that are
not used over successive generations will degenerate and
disappear.
• Although it recognizes that species change over time it is not a
good mechanism for explaining change because it does not state
how those changes occur.

Organic evolution
• The Evolutionary Theory
– Charles Darwin (1809-1882) developed the
Theory of Natural Selectionin 1859.
• It was superior to Lamarck ‘s idea because it provided a
mechanismto cause the changes.

The Protochordates
Chapter III

Introduction
of
Protochordates

Protochordates
• The term Protochordata is used to include to
phyla of the chordates.
• These two phyla are:
– Urochordata – the sea squirts and other loosely
related groups.
– Cephalochordata – represented by amphioxus.
• Both phyla are marine organisms that most likely share
a common ancestor with vertebrates.

Protochordates
• Protochordates, vertebrates, and the most likely
invertebrate relations the echinoderms and
hemichordates can be collectively grouped under the
heading Deuterostomata.
• Deuterostomata means “two openings” and refers to
an event during embryological development.
– During embryogenesis a blastopore develops.
– The blastopore develops into the anus and a second
opening will form during development that will become
the mouth.
– This pattern of development indicates the possibility of a
common ancestor for these disparate groups.

Echinodermata
• The echinoderms first arose during the Cambrian
Period and include 20 extant classes such as sea
stars, sea cucumbers, and sea urchins.
– All echinoderms have a calcium carbonate skeleton,
are marine, and most species show radial symmetry.
– However bilateral symmetry is a primitive
characteristic of echinoderms.
• The relationship between echinoderms and
vertebrates has been demonstrated by
embryogenesis.

Hemichordata
• Hemichordates are a group of marine
invertebrates commonly called “acorn worms”.
• The relationship between the hemichordates
and the vertebrates is widely disputed. The
hemichordates are a unique taxon.
– Supporters of a close relationship cite the
following items as evidence:
• Hemichordates have a structure that resembles a dorsal
hollow nerve cord.
• It is a dorsally oriented strand of nerves that
occasionally possesses a lumen.

Hemichordata
• There are two problems with this, however:
– The presence of the lumen is highly variable.
– Hemichordates also have a ventral nerve cord (as do many
invertebrates).
– . Hemichordates lack a true CNS.
• Hemichordates have slits in their pharynx. These slits
represent gill slits and may be pharyngeal slits.
• Hemichordates possess a structure called the Stomochord.
– The stomochord may be homologous with the notochord.
– The relationship between the stomochord and notochord lacks
evidence.
– It is a short outpocketing of the foregut into the proboscis of
the acorn worm.

Hemichordata
• Developmental and molecular biological
studies indicate a closer relationship exists
between hemichordates and echinoderms
than between hemichordates and vertebrates.
– The larval forms are very similar.
– Muscle proteins and other traits are close
between the two.

Urochordata
• Urochordates are a group of filter feeding,
marine organisms.
– Urochordates typically have a thin, transparent,
nonliving outer tunic.
• For this reason they are often also known as the tunicates”.
– Since they possess a notochord in their tail, tunicates
are considered to be chordates.
– There are three classes of Urochordates: Ascidians,
Larvaceans , and Thaliaceans.
• Two of the three classes retain the larval form throughout
life.
• Only the ascidians mature into an adult form. They lose the
tail and its notochord.

The Class Ascidians
• Ascidians are tiny (between 0.5mm to 11 mm),
short living (hours to days) organisms found
either in colonies or existing solitarily.
• The larval form consists of a trunk and a muscular
tail.
– The tail is locomotory and possesses a notochord.
• Running along the notochord are between 36 and 1,600
uninucleated, striated muscle fibers.
– The larval form does not feed.
– As a result the viscera is reduced.
– The larval form feeds off of stored calories.

The Class Ascidians
• The larval form’s nervous system consists of a dorsal, hollow nerve
cord, several ganglia, and nerves.
• It also has a primitive brain composed of a few ganglia with
associated sensory structures. These structures are:
– Ocellus, a light sensitive receptor cell
– Otolith, a receptor for stereoreception.
• During respiration, water is drawn into the mouth and then passes
through the pharynx and over the gills lining the pharyngeal slits.
– Then it passes into a chamber surrounding the pharynx called the
Atrium .
– Oxygen depleted water will exit the atrium via the Atriopore.
– When the larval form metamorphosis into the adult form the following
events occur:
• Three adhesive papillae form to attach the sessile adult to a substrate.
• The notochord is reabsorbed and used as a source of nutrition to supply the
energy required for metamorphosis.

The Class Ascidians
• During respiration, water is drawn into the mouth
and then passes through the pharynx and over
the gills lining the pharyngeal slits
– The viscera changes.
• The mouth becomes and incurrent siphon and the
atrioporebecomes and excurrent siphon.This modification
allows for filter feeding.
• A ciliated, glandular groove develops on the floor of the
pharynx called the Endostyle.The endostyle traps food and
passes it on to the esophagus by ciliary beating.
• The ascidian also develops a short esophagus, stomach, and
intestine.
• Wastes are dumped into the atrium and released along with
deoxygenated water by means of the atriopore.

The Class Larvaceans
• Larvaceans typically occur as free - floating
plankton.
• They are found in waters of less than 100m
depth and below the level of light penetrance.
• They are tiny being about 8mm in length.
• They have a small body and a long tail
supported by a notochord.

The Class Thaliceans
• Thaliceans are free living or colonial
organisms.
• They alter between free living and colonial
over succeeding generations.
• Thaliceans resemble adult ascidians in
appearance.
– Like adult ascidians they lack a tail.
– They propel themselves by squirting water out of
the atriopore.

Cephalochordates
• Represented by Amphioxus.
• AKA lancelets due to the
appearance of their cylindrical
bodies which taper at both
ends.
• Small, marine organisms found
immediately off of sandy
beaches.
• They burrow into the sand leaving
only their heads exposed so as to
filter feed.
Figure . The Amphioxus

Amphioxus (Anatomical Features)
• Body is consists mostly of a trunk.
– The length of the trunk has repeating units of
muscle termed Moyer`s that serve for locomotion.
• The myomeres are composed of uninucleated, striated
muscle fibers.
• Adjacent myomeres are separated by a thin connective
tissue partition called a Myoseptum/Myocomma

• It has a semitransparent skin composed of two
layers.
– There is an epidermis composed of a single layer
of cells and a thin dermis that facilitates
cutaneous respiration.
– The integument also includes a thin hypodermis.

• Possesses pharyngeal slits.
– The pharyngeal slits do not open directly to the
exterior.
• Instead they open into the Atrium , a fluid - filled cavity
that surrounds the pharynx.
– Food cleansed waters are pumped from the pharynx into the
atrium and expelled out of the body by a ventral opening called
an Atriopore.
– The slits do not serve a respiratory role but instead help to
facilitate filter feeding.
– Amphioxus uses the skin for respiration.
– The number of pharyngeal slits can number up to 60.
– They are reinforced by bars of cartilage termed Pharyngeal
Bars.

• Amphioxus has a notochord that runs from
the rostrum to the tip of the tail.
– Unlike the typical notochord, the notochord of
amphioxus consists of sequentially arranged
muscular discs.
• These muscular discs have fibers that anchor the discs
to a surrounding notochord sheath composed of
fibrous connective tissue
• Muscular contractions increase the stiffness of the
notochord that may help in digging.

• The nervous system of amphioxus is similar to that
of vertebrates.
– It has a CNS consisting of a brain and spinal cord.
• The CNS is covered by a vascular membrane called the
Leptomeninx
• The brain of amphioxus has two subdivisions.
– Radiating from the spinal cord are spinal nerves.
– Amphioxus does possess special senses but they are rudimentary.
• Chemoreceptors are scattered over the surface of the
body.They are particularly concentrated in the anterior
pharyngeal region and mouth.
• There also are light sensitive pigmented ocelli embedded
along the length of the spinal cord in the venterolateral walls.

• Amphioxus feeds by filtering food out of
the water.
–Anterior to the mouth is the Vestibule , a
structure designed o collect seawater.
• The vestibule is surrounded laterally by the Oral
Hood.
• Caudally there is a perpendicular membrane
called the Velum.
– The velum is perforated by the mouth.
– The mouth opens into the pharynx.

• Cilia on the pharyngeal bars pull water through the
vestibule, through the mouth, and into the pharynx.
– Within the vestibule is a mucus covered structure called
the Wheel Organ.
• The wheel organ has stubby projections to help trap food missed by
the mouth.
• The opening of the vestibule is lined by Buccal Cirri
which serve to partially strain incoming water.
– The buccal cirri have chemoreceptor's to monitor incoming
water.
– Once food is in the pharynx it will be processed

• The processing of food requires specialized
features of the pharynx:
– The pharynx has a ventral invagination lined with cilia
and mucus called the Endostyle or Hypobranchial
Groove.
– There is also a dorsal invagination lined by cilia and
mucus termed the Epibranchial Groove.
– Connecting these two grooves are Pharyngeal Bands.
• Pharyngeal bands cover the pharyngeal bars and are covered
by mucus and cilia.
• Mucus secreted by these structures trap food.

• The mucus and food become sticky mucus
strands that are propelled by ciliary beating
into the epibranchial groove and then pass
caudally into the gut..
– In the gut digestive enzymes will be added to the
food strand.
– Ciliary beating pushes the food – enzyme-mucus
mixture into theintestine.
• Some of the mixture is passed to the cecum for the
introduction of more enzymes.
• The intestine terminates at the anus.

• The coelom is reduced in adult
amphioxus.
–It becomes laterally compressed as the
larvae elongates and more pharyngeal bars
are added.
–As a result, only a trace coelom is found in
the adult.

• Amphioxus circulation differs significantly from
that of vertebrates.
– It lacks a heart and formed elements
– The blood is a colorless serum.
• One similarity to vertebrate circulation is that the
amphioxus venous pathways resemble those of
embryonic vertebrates.

• Cephalochordates lack an organized
kidney.
–Instead individual cellular components
called Cyrtopodocytes filter the blood.
–Cyrtopodocytes are intermediary between
the Protonepheridia of invertebrates and
the podocytes of vertebrates.

• In mature amphioxus, the gonads will be
prominent and will bulge into the atrium.
– The gonads release sperm or eggs into the atrium.
• The gametes are released from the atrium by the atriopore.
– Amphioxus are Dioecious meaning that they develop
only ovaries ortestes.
– Amphioxus resembles the larval form of the lamprey
called the Ammocoete.
• Initially the ammocoete larvae were actually classified as
protochordates.
• One reason is that they stay in the larval state for a long time
(between 2 to 6 years).

Ammocoetes VS. Amphioxus
• Ammocoetes have a notochord running from
their midbrain into the tail.
• Like amphioxus ammocoetes have a dorsal,
hollow nerve cord and a brain.
– Development of the nerve cord is the same for both.
– One difference is that the ammocoete brain has three
vesicles.
– Ammocoetes have discrete special sense organs
although they are rudimentary by the typical
vertebrate standard.

The Agnathans
• The vertebrates can be divided up into two super
classes based on the presence or absence of jaws.
– The jawed vertebrates are called the Gnathostomata.
• b) The vertebrates lacking jaws are called the
Agnathostomata.
– Agnathans are considered to be the most primitive
vertebrate taxon.
– Today only two extant groups exist; lampreys and
hagfish.

The Agnathans
• Although there are only two extant groups
today, there are a large number of extinct
agnathans called the Ostracoderms.
– Ostracoderms were a group of armored fish- like
organisms of the late Cambrian through the early
Devonian periods.
– Ostracoderms are the oldest known vertebrates
with fossils dating back to the Ordovician period
and they are expected to have existed prior to
that during the late Cambrian.

The Agnathans :Ostracoderm
Anatomy
• Ostracoderms were typically small marine species
being between 2 to 3 cm in length.
– However some species grew up to 1 m in length.
• They are characterized by lacking jaws, lacking paired
fins, and having a body composed of overlapping plates
of dermal bone.
• The plates were largest on the head forming a bony
shield and smaller, more tile - like bones, on the trunk
and tail.
• Ostracoderms had three eyes.
– There was a pair of upwardly oriented orbital eyes.
– There was a third singular eye called the Pineal Eye.

The Agnathans :Ostracoderm
Anatomy
• They had a singular naris (nostril) which
opened into a Nasopharyngeal Duct
connecting to an olfactory sac.
• They had a small, jawless mouth.
• They had rows of external gill slits.
• Ostracoderms , along with the dermal bone,
also had an endoskeleton composed of bone
and cartilage.

The Agnathans:Extant Agnathans
• The extant agnathans are hagfishes and lampreys.
• The two groups are not really closely related and
occupy two separate classes.
– Hagfishes belong to the class Mixini.
– Lampreys belong to the class Cephalospidomorphi that also
includes the ostracoderms.
• Extant agnathans share some common characteristics:
– They have a prominent notochord that forms the axial
skeleton throughout the agnathan`s life.
– They lack a bony skeleton, scale, or dermal bony plates.
• They lack bony jaws.
• They lack a vertebral column.
• These missing features may actually be derived characteristics that
evolved to maximize a life as a parasite.

The Agnathans:Extant Agnathans
• Agnathans have fewer semicircular canals
than do gnathans (which have three).
–Lampreys have two semicircular canals.
–Hagfish have only one.
–They had a singular naris that opened into a
nasopharyngeal duct connecting to an
olfactory sac.
–The mouth is surrounded by a Buccal
Funnel.

The Agnathans: The Hagfishes; Class
Mixini
• Hagfish are a class of marine, eel - like organisms.
• They are bottom dwelling scavengers.
• Hagfish have shallow buccal funnel surrounded by
finger – like papillae resembling short tentacles.
• They have vestigial eyes covered by a thin, opaque
membrane.
– Lampreys have better developed eyes.
• The number of pairs of gill slits varies between
species.

The Agnathans: Lampreys
• Lampreys resemble hagfish in appearance being eel - like
aquatic organisms.
• Most species are marine but some spend at least part of
their lives in freshwater.
• Lampreys have changed very little from the time of the
Carboniferous.
• All lampreys are parasitic, feeding off of the blood of fishes.
– They have a well developed buccal funnel with tooth - like
structures that anchor the lamprey on to its prey.
– Lamprey have a muscular tongue coated with tooth - like
denticlesdesigned to scrap away the flesh of the prey so as to
feed on its blood.

The Agnathans: Lampreys
• Like ostracoderms, lamprey have a singular
naris that opened into a nasopharyngeal duct
connecting to an olfactory sac.
– Unlike ostracoderms, in lampreys the olfactory sac
terminates blindly into a Nasohypophyseal Sac.
• All lamprey species have seven pairs of gill
slits.

The Vertebrates
Superclasss
Gnathostomata

Class Placoderm
• Placoderms are the earliest of the jawed
vertebrates existing during the Paleozoic Era and
primarily during the Devonian Period.
– They are extinct.
– They evolved from the ostracoderms.
• Placoderms were named for their bony dermal
plates.
• They had paired fins; both pelvic and pectoral.
• Placoderms species ranged in size from a few
inches in length to a species that reached 20 ft.

Class Chondrichthyes
• Chondrichthyes is an ancient class and has two extant
subclasses; Elasmobranchii and Holocephali.
• Elasmobranches have a cartilaginous skeleton.
– The only bony component is found in the teeth and placoid
scales.
• Male elasmobranches have structures called Claspers which
serve as intermittent organs.
– Elasmobranchs are the sharks and rays.
• They first arose in the late Silurian Period and continue to exist today.
• They have exposed gill slits, typically five pairs, and one pair of
spiracles.
• Holocephalians are the chimera (aka; rat fishes).
– They differ from elasmobranches in: an operculum that covers thegill slits,
and the teeth are replaced by bony plates.

Class Teleostomi: Acanthodians
• The acanthodians are a group of fishes named
for their hollow spines.
– Along with spines, bony plates also cover the
body.
– They had an operculum.
– The skeleton had both cartilaginous and bony
elements.
• They are extinct having died out in the
Carboniferous Period.

Class Teleostomi: Osteichthyes
• The osteichthyes are the ray- finned fishes.
– They include both ancient and modern bony fishes.
• The osteichthyes can be divided into the
Actinopterygii and Sacropterygii.
• They first arose sometime between the early
Devonian and late Silurian.
• The group is named for their bony skeletons
(although that is not true for all).
– They have membranous fins supported by bony rays.
– They have an operculum.
– They have scales.

Class Osteichthyes : Subgroup
Actinopterygii
• Subclass Chondrostei
– This is a primitive group of osteichthyes have few
extant species.
– Today represented only by the paddlefish and
sturgeons.
– They have an endoskeleton composed mostly of,
but not exclusively of, cartilage.

Actinopterygii
• Subclass Neopterygii
– The Gars and Bowfins
– This division represents another primitive group of
fishes having few extant species, the gars and
bowfins.
– Unlike the chondrosteins, their skeleton is mostly
ossified.
– A distinct feature of this div ision is their scales
that are Ganoid scales.

Actinopterygii
• Division Teleostei
– The teleosts are the modern fishes.
– They are the most successful group of vertebrates
today.
• The teleosts make up 96% of all extant fish species and
have three suborders.

Sarcopterygia
• This is a group of bony fishes having a fleshy
lobe to their paired fins.
• The sarcopterygians gave rise to the
tetrapods.
• They have internal nares that open internally
into the oropharyngeal cavity.
• They retain a gas filled air sac that has the
potential to develop into a lung.

Sarcopterygia
• The sarcopterygians have two orders:
–Actinistians, the coelacanths
–Rhipidistians, the lungfishes.
• This order includes the suborder Dipnoans
which is the group that gave rise to amphibians.

Class Amphibia
• The amphibians are the first tetrapods.
• They first appeared in the late Devonian and
began to radiate in the Carboniferous.
• The class can be divided into two subclasses:
– Labyrinthodontia were a group of early extinct
amphibians.
– Lissamphibiaare the modern, extant amphibians.

Class Amphibia: Subclass
Labyrinthodontia
• The labyrinthodonts were the first amphibians.
– The oldest known representative was Ichthyostega that
appeared in the late Devonian.
• Anatomical Features:
– The labyrinthodonts are a diverse group but all share a
similar patternon the apical surfaces of their teeth (which
gave the subclass its name).
– They share many features with rhidipstian fishes such as:
– dermal scales, a fish - like tail supported by ray, rhidipstian -
like skull and the presence of a lateral line, at least on the
skull.
• Today w see lateral lines in the tadpole but it is lost in the adult form
of extant groups.

Labyrinthodontia
• One group of labyrinthodonts was the
Anthracosaurs.
– This group is believed to have given rise to amniotes
in general and to reptiles in particular.
– They included the Seymouria that is believed to have
laid in leathery eggs.
• Another group of labyrinthodonts were the
Temnospondyls.
– Temnospondylian skulls display some features found
in modern amphibians (especially frogs) and is
believed to be the ancestor of the Lissamphibia.

Lissamphibia
• These are the modern amphibians. There are
three groups.
– Order Anura/Salientaare the frogs and toads.
• Anuran fossils date back to the Triassic.
– Order Urodela/Caudataare the salamanders.
– Order Apoda/Gymnophiona are the caecilians.
• This is a group of tropical, limbless amphibians.
• They can either be aquatic of burrowing.
• The apoda may have a different origin than do the rest
of theLissamphibia.

Class Reptilia
• The reptiles are the first Amniotes.
– Amniotes include all of the mammals, birds, and reptiles.
• Amniotes are characterized by:
– the development of shell - covered eggs
• This trait can be replaced over evolutionary time by becoming live -
bearing.
• This trait increased the ability of tetrapods to conquer land.
– possession of extra embryonic membrane such as the amnion.
• These membranes allowed reptiles to lay eggs on land and
avoid an aquatic larval stage. They are:
– The Amnion is a fluid filled sac that surrounds the embryo.
– The Allantois
– The Chorion

Class Reptilia
• Along with the development of the amniotic egg,
reptiles show other features that increased
terrestrial adaptiveness.
– They are covered by scales composed of cornified
epidermal scales so as to decrease dehydration.
– They developed a neck.
– They have better developed limb girdles and claws on
their digits.
– They developed the Metanepheric Kidney.
– They display the partial or complete division of the
heart into right and left chambers.

Class Reptilia
• Modern and ancestral reptiles are Ectothermic.
• However they have given rise to endothermic tetrapods on a
few occasions such as mammals, birds, and possibly some
dinosaurs.
• Reptiles developed from a labyrinthodont, most likely an
Anthracosaur, during the early Carboniferous.
– Based on skull structure, two reptilian lineages arose from
a common ancestor.
– Synapsids are the lineage that gave rise to mammals.

Class Reptilia
• The skull possesses one temporal fossa on
each side.
• They are believed to have evolved separately
from the other reptiles.
• Reptilia(or true reptiles or Sauropsida are all
the modern reptiles, the dinosaurs, and gave
rise to the birds.

Class Reptilia
• Reptilia can be divided into two subgroups based on skull
structure:
– Anapsids are reptiles whose skulls lack temporal fossa.
• This is a primitive condition found in basal reptiles.
• Today the only extant representatives are the turtles.
• Anapsids gave rise to the diapsids.
• Diapsids are all of the other reptiles.
– Diapsid skulls have two temporal fossa on each side.
– Diapsids are all of the extant reptiles (except for turtles) and many
extinct species. They include:
• Squamates; the snakes and lizards.
• Archosaurs; the dinosaurs, pterosaurs, and crocodilians
– The diapsid reptiles gave rise to Euryapsid Reptiles that have lost
one pair of temporal fossa (only one temporal fossa on each side).
Euryapsids were the plesiosaurs and ichthyosaurs.

Class Aves
• Aves are bipedal, endothermic amniotes that are
covered with insulating feathers and have evolved for
flight (although some have lost this trait).
– The origin of birds is still controversial.
– The prevailing hypothesis today is that they evolved from
small theropod dinosaurs or share a common ancestor
with them.
• Both groups share numerous anatomical features.
• Some even classify birds as reptiles of the group
Archosauria today.

Class Aves
• Along with the previous mentioned traits,
birds have evolved other traits for flight.
– lightweight bones many of which possess air sacs
– loss of teeth
– modified forelimbs
• There are two generally recognized
subclasses:
– Archaeornithesare the extinct, ancient birds (ex;
archaeopteryx).

Class Aves
– Neornithes are the modern birds. There are
three orders:
• Odontognathaeare a group of extinct, toothed
marine birds.
– There only two known representatives ichthyrinis and
hesperornis.
• Palaeognathae(aka; ratites) are a group of flightless
birds (ex; ostriches and rheas).
• Neognathae(aka; carinates) are all other modern
birds.
– Neognathans have a large keel, or carina, for anchoring
flightmuscles.

Class Mammalia
• Mammals are endothermic amniotes covered by
insulating hair and producing milk for their young.
• Mammals evolved from synapsid reptiles.
• Synapsid reptiles diverged from all other reptiles
very early inreptilian history (in the early
Carboniferous).
– The earliest synapsid reptiles were the Pelycosaurs.
– The pelycosaurs evolved into the Theraspids that gave
rise to mammals.
– There are a number of features found in modern
mammals that were present in therapsids:
• such as 2 occipital condyles, a secondary palate, and
heterodontic teeth.

Class Mammalia
• Mammals first appeared in the Triassic period.
• Modern mammals still display a synapsid skull.
• Other features of mammals include:
– 3 auditory ossicles
– a muscular diaphragm separating the thoracic and
abdominopelvic cavities
– absence of a cloaca in the adult in all mammals excepting
themonotremes
– the vast majority possess sweat glands
– presence of heterodontic teeth with adult and juvenile dentitions
– The homodontic teeth of cetaceans are a derived characteristic.

Class Mammalia
• Other features of mammals include:
– loss of the fourth aortic arch
– a single dentary bone on each side of the lower jaw that
willarticulate with the squamosal bone
– pinna to gather sound waves
– a more specialized larynx
– extensive development of the cerebral cortex.

Class Mammalia
• Based on reproductive strategy there are two groups of
mammals.
– Prototheria (or Monotremes) are egg laying mammals.
– Metatheria are mammals that give rise to live young. There
are two groups:
• Metatheria(or Marsupials) give birth to almost larval
young that migrate to, and finish development in a
pouch.
– They lack a placenta.
– Instead a yolk sac serves as the role of the placenta.
• Eutheria (or Placental Mammals) have young that
receive nourishment via a placenta while in the womb.

Development of
the Vertebrate
Body
Chapter V

Vertebrate Eggs
• Vertebrate eggs vary in the amount and location
of their yolk.
– The amount of yolk present causes vertebrate eggs to
be divided into 3 categories:
• Microlecithal Eggsare eggs having very little yolk.
– Microlecithal eggs are found in amphioxus and therianmammals.
• Mesolecithal Eggs are eggs having a moderate amount of
yolk.
– Mesolecithal eggs are found in freshwater lampreys,
actinopterygians, gars & bowfins, lungfishes, and amphibians.
• Macrolecithal Eggsare eggs having a lot of yolk.
– Macrolecithal eggs are found in marine lampreys, teleosts,
reptiles, birds, and monotremes.
– Yolk distribution occurs in two patterns in vertebrate eggs:

Vertebrate Eggs
• Isolecithal is the pattern where the yolk is evenly
distributed throughout the egg.
– The isolecithal pattern tends to be found in eggs having a
microlecithal yolk content.
– Isolecithal eggs are found in therian mammals.
• Telolecithal is the pattern where the yolk is unevenly
distributed.
– This results in the egg being divided into two poles.
• The Vegetal Pole is a large yolk mass.
• The Animal Pole contains the embryo.
– The telolecithal pattern is found in eggs having a
macrolecithal or a mesolecithal yolk content.
– Telolecithal eggs are found in all nontherian vertebrates (
including amphioxus).

Oviparity and Viviparity
• Oviparous animals lay eggs.
– The eggs contain all of the nourishment necessary
for the development of the offspring.
– If there is not enough yolk, as is the case with
amphibians, the young will hatch in a larval form.
• Viviparous animals give birth to live young.
– There are two patterns of viviparity based on the
source of nourishment.

Viviparity
• Ovoviviparous - nourishment comes form yolk as in
oviviparous animals.
– The egg hatches shortly before birth or very shortly after birth.
• The mother`s body provides protection for the egg but does not
provide nourishment or oxygen.
• Found, for example in Squalus sulkeyi.
• Euviviparous – nourishment is supplied by the mother.
– Maternal tissues supply nourishment and oxygen and also
remove wastes.
• ex; placental mammals
• There are a variety of intermediary forms.
– For example, some reptiles nourish the embryos with yolk early
in the pregnancy and then by maternal tissues later in the
pregnancy.

Fertilization
• Fertilization can be either external or internal.
– Internal fertilization occurs in species that are viviparous or
are oviparous species that produce hard shelled eggs.
• The male will have an organ designed to introduce sperm into the
female reproductive tract called an Intromittent Organ.
• Oviparous animals that produce soft shelled eggs will use external
fertilization.
– External fertilization occurs in most fishes and anuran
amphibians.
• One exception are rodeles and apodans which utilize internal
fertilization even in oviparous species.
• Due to the needs of external fertilization, reproduction will take
place in an aquatic environment.
• To compensate for the diluting effects of water, a copious amount
of sperm is released over the eggs as they are being laid.

Early Development in Representative
Chordates
• From Zygote Stage Through the Blastula Stage
–The Zygote is the fertilized egg, the first
embryonic stage.
–Once fertilization has occurred the zygote
immediately begins to undergo a special
form of cell division termed Cleavage or
Segmentation.
–Unlike typical cell division, cleavage lacks a
resting period between mitotic events
meaning that there is no time to allow for
the recovery of cytoplasm.

Early Development in Representative
Chordates
– As a result, each time the cell divides, the two
daughter cells are smaller than was the mother cell.
– So the eight celled embryonic stage is roughly the
same size as is the one celled zygote.
– The embryo will become subdivided into smaller and
smaller cells until forming a hollow ball of cells termed
the Blastula.
– Each cell of a blastula is termed a Blastomere and the
hollow center of the blastula is called the Blastoceol.

Patterns of Blastula Development
• In microlecithal eggs the blastomeres are of approximately
equal size.
• In mesolecithal eggs the increased yolk content (compared
to microlecithal eggs) interferes with cleavage resulting in
blastomeres of unequal size.
– Cells at the animal pole will divide faster than will those at the
vegetal pole.
– As a result, the cells of the animal pole will divide faster and be
smaller than will those of the vegetal pole.
• In macrolecithal eggs there is even a higher yolk content
causing even more interference with cleavage.
– As a result, cleavage is restricted to the animal pole.
– This results in a group of cells called a Blastoderm resting on a
large aggregate of yolk.

• Therian mammals have microlecithal eggs with very little yolk
and do not show the typical vertebrate pattern of animal and
vegetal poles.
– As a result therian mammals show a unique cleavage
pattern.
– Cleavage starts out as in other vertebrates but will show
divergences as it progresses.
– So the blastula in therians is given a special term, the
Blastocyte.
• Cleavage will result in a group of cells located within the
blastocoel appearing during the 16 cell stage called the Inner
Cell Mass.

• By the 32 cell stage the outer layer of the embryo has
expanded to form the Trophoblast.
– The trophoblast can absorb uterine secretions to
nourish the inner cell mass prior to the
establishment of the placenta.
– The trophoblast will develop into the placenta and
the chorion.

Gastrulation
• Gastrulation is a series of cellular movements
in the embryo resulting in the formation of the
three primordial germ layers.
– The three primordial germ layers are: Ectoderm,
Mesoderm , and Endoderm.
– During gastrulation the embryo will go from a
hollow ball of cells to a three layered embryonic
stage in which bilateral symmetry is established.
– These cellular migrations are called Formative ,or
Morphogenetic, Movements and are controlled by
chemical agents termed Morphogens.

Gastrulation in Amphioxus, An Animal
with a Microlecithal Egg
• Beginning with the blastula: a group of
blastomeres, termed Presumptive Endoderm
,invaginates into the blastocoel.
• This process is called Involution.
• Involution causes the formation of the
Archenteron, the primitive gut.
– This embryonic stage is called the Gastrula.
– The entrance into the archenteron is termed the
Blastopore.
– Cells from the blastopore migrate into the gastrula to
begin the formation of the notochord.

• As a result, these presumptive endodermal cells
are termed Presumptive Notochord Cells.
• The early forming notochord will temporarily
form the roof of the archenteron.
• Presumptive Mesoderm Cells will begin to
aggregate lateral to the newly forming notochord
and become undifferentiated mesoderm.
• At this stage the amphioxus embryo is composed
of an outer layer of ectoderm and an inner layer
of endoderm surrounding the archenteron.

– The roof of the archenteron has the notochord flanked by
undifferentiated mesoderm.
– Once the mesoderm has become established, the
undifferentiated mesodermal cells will be arranged in bands.
• These mesodermal bands will fold and move upwards due
to the driving force of mitosis that is still occurring rapidly.
• The upwardly folding mesoderm will form Mesodermal
Pouches(aka; Coelomic Pouches posterior of the animal.
– Each mesodermal band will produce its own mesodermal
pouch.
– At this point, the amphioxus larva has hatched and is swimming
about in its environment.

• Once the mesodermal pouches form, they begin to extend ventrally
between the ectoderm and endoderm.
– The right and left mesodermal pouches will eventually extend to and
meet at the ventral aspect of the larva to form the coelom.
• The fate of the mesoderm in the mesodermal pouches:
– The outer wall of each pouch lays against the ectoderm and is now
termed Somatic Mesoderm.
– Together with the ectoderm somatic mesoderm makes up the body
wall or Somatopleure.
– The inner wall lays against the endoderm and is now termed
Splanchnic Mesoderm.
– The coelom is the space between the splanchnic and somatic
mesoderm.

• The development of the mesodermal pouches occurred in a
segmental pattern in amphioxus.
– The pouches on the right and left hand sides will fuse
ventrally.
– Later the fused pouches will begin to fuse to one another
between adjacent segments starting anteriorly and
progressing posteriorly.
• The germ layers are now established and Organogenesis (the
formation of organs and organ systems) can occur.

Gastrulation in Frogs, An Animal with
a Mesolecithal Egg
• In vertebrates other than therian mammals yolk in the
vegetal pole affects Gastrulation.
– In frogs a process called Epiboly occurs.
• The small, rapidly dividing cells of the animal pole will
grow downwards over the large, slower dividing, cells of
the vegetal pole.
• This group of rapidly dividing cells will flow inwards, over
the yolk forming a blastopore and an archenteron. This is
called Involution.
– The yolk cells will make up the floor of the
archenteron.
– The rest of embryogenesis is similar to that seen in
amphioxus .. .

Notochord and Dorsal Mesoderm
Formation in Frogs/MesolecithalEggs
• As involution progresses, a stream of
undifferentiated cells will flow over the
dorsal lip of the blastopore.
• These cells establish a narrow band on the
roof of the archenteron called the
Chordomesoderm.
– A notochord develops from the midline of the
chordomesodermand will temporarily remain on
the roof of the archenteron.

Notochord and Dorsal Mesoderm
Formation in Frogs/Mesolecithal Eggs
– The chordomesoderm cells lateral to the
developing notochord will give rise to a pair of
undifferentiated mesodermal bands (lateral to the
notochord).
• These bands of mesoderm form the dorsal
mesoderm.
• The dorsal mesoderm will become segmental
and form hollow mesodermal somites.
– The somites will form the musculature of the body
wall.

Endoderm and Ectoderm Development
Formation in Frogs/Mesolecithal Eggs
• While the notochord and dorsal mesoderm
are developing, presumptive endodermal cells
will migrate into the interior of the embryo,
spread out, and provide (with the yolk cells)
the lining of the archenteron. These cells will
be the endoderm.
• The cells on the surface of the gastrula
become the ectoderm.

Intermediate and Lateral Plate Mesoderm
Development Formation in
Frogs/Mesolecithal Eggs
• Mesodermal cells (other than those in the
somites) will migrate into the gastrula through
the blastopore.
• These mesoderm cells expand out as a sheet
moving cephaladbetween the ectoderm and
the endoderm.
– This mesoderm is termed Lateral Plate .

Development Formation in
Frogs/Mesolecithal Eggs
– Lateral Plate Mesoderm will divide into two
sheets:
• Somatic Mesodermwill be the outer sheet.
• Splanchnic Mesoderm will be the inner sheet.
• The coelom is the space between the somatic and
splanchnic mesoderm.

Development Formation in Frogs/Mesolecithal
Eggs
• Somatic mesoderm and the ectoderm together
form the somatopleure.
• Splanchnic mesoderm and the endoderm
together form the splanchnopleure.
• A portion o the lateral plate mesoderm does not
follow this pattern.
– This portion of the lateral plate mesoderm is located
dorsal to therest of the lateral plate mesoderm and
immediately lateral to the somites.
• These cells will give rise to much of the urogenital system.
• This mesoderm is called Intermediate Mesoderm or
Nephrogenic Mesoderm.

Gastrulation in Birds, An Animal With
a Macrolecithal Egg
• Due to the greater yolk content of macrolecithal eggs,
epiboly cannot occur.
– Remember cleavage does not occur in the yolk in macrolecithal
eggs.
• Only the cells of the animal pole will demonstrate cleavage and give rise
to a Blastoderm.
– The blastoderm organizes into an upper sheet of cells
(theEpiblast) and a lower sheet of cells (the
– Hypoblast) separated by the blastocoel forming a Bilaminar
Embryonic Disc.
• Cells from the periphery of the hypoblast will grow downward overthe
yolk sac.
• They will become part of the endoderm lining the yolk sac.
• The hypoblast cells do not become a portion of the developing embryo.
– Instead the hypoblast cells are limited to the yolk sac and its stalk.
– The cells of the epiblast will give rise to all of the structuresof the
developing embryo.

Notochord and Mesoderm Formation
in Birds/Macrolecithal Egg
• Cells from the epiblast develop into a Primitive Streak.
– The primitive streak is a thin, multilayered structure running the length
of the embryo.
• A thickened nodule of closely packed blastoderm cells forms along
the primitive streak called Hensen's Node.
– Hensen`s node defines the caudal end of the future embryo.
• The primitive streak and Hensen`s node are the functional
equivalent of the blastopore in eggs having less yolk.
• Epiblast cells migrate from the primitive streak into the
hypoblastwhere they will displace the hypoblast cells to establish
the endoderm.
– Epiblasts cells will then migrate a second time. These cells will situate
between the epiblast and endoderm to form the mesoderm.
– The Remaining portion of the epiblast is now the ectoderm.
– The result of these migrations is the formation of a Trilaminar
Embryonic Disc.

A Notochord Process develops from the
primitive steak and pushes through the
epiblast.
• Other cells move forward along the newly forming
notochord to become the dorsal mesoderm.
– Simultaneously the lateral plate mesoderm streams in between
the ectoderm and endoderm.
– The dorsal mesoderm will eventually segment to form somites.
• Simultaneously the lateral plate mesoderm differentiates into somatic
and splanchnic mesoderm.
• The splanchnopleure will extend down over the yolk to form
a yolk sac.
– By the second day of incubation the splanchnopleure produces
the first blood cells and a delicate network of Vitelline Vessels
(aka; Omphalomesenteric Vessels) on the Area Opacaof the yolk
sac.
• Vitelline vessels carry yolk globules into the embryo.
– Carries yolk to the early, “S” shaped heart of the embryo for further
distribution.

Gastrulation in Placental Mammals
• Gastrulation begins with the Blastocyte, a hollow
ball of cells havinga blastocoel and being divided
into a trophoblast and an inner cell mass.
– The trophoblast does not contribute to the embryo, It
contributes to the extraembryonic membranes.
• It will establish contact with and implant in the uterine wall.
– The cells of the inner cell mass will develop into
embryonic structures.
• Using a variety of mechanisms in different
placental mammals, the inner cell mass will
divide into a Bilaminar Embryonic
Disc/Blastodiskhaving an epiblast and primitive
endoderm.

• Then epiblast cells will migrate into the space
between the epiblast and primitive endoderm to
form the mesoderm and a Trilaminar Embryonic
Disc.
– The epiblast cells at this point will become ectoderm.
– Mesoderm cells will develop into dorsal mesoderm,
forming along the developing notochord, and lateral
plate mesoderm.
• 1] The lateral plate mesoderm will develop into somatic and
splanchnic mesoderm.

• Notochord formation is similar in mammals as
it is in macrolecithal eggs which demonstrates
our reptilian ancestry.
• In placental mammals the three gem layers
begin to form at the same time as the
extraembryonic membranes begin to form.

Neuralation
• Neural tissue develops from the ectoderm.
– Presumptive neural ectoderm I found in a wide
band on the dorsal aspect of the embryo laying
above the notochord.
• As the gastrula elongates, this band thickens forming
the Neural Plate/Floor Plate.
• The neural plate develops a pair of folds called Neural
Folds on either side of the Neural Groove.
• The formation of the neural folds vs. the neural groove
is due to different mitotic rates.
• At this point the embryo is now termed a Neurala.

Neuralation
• The neural groove is widest at the anterior extremity of
the gastrula.
– This wider region will become the brain.
– The remainder of the neural groove will form the spinal cord.
• The neural plate will sink deeper into the neural groove
and the two neural folds will grow closer together.
• Eventually the neural folds will meet over the neural
tube and fuse together.
• `Now the neural groove is a Neural Tube.
• The cavity enclosed by the neural tube is called the
Neurocoel.
– The neurocoel will give rise to the ventricle system of thebrain
and the central canal of the spinal cord.

Neuralation
• As a result of the developmental processes
involved, the CNS is ectodermal, dorsal, and
hollow.
– Neuralation will vary a bit between different
vertebrates. The differences are minor overall.
• One of the greatest differences is seen in agnathans and
actinopterygians where neural folds do not form.
• Instead the neural ectoderm becomes a wedge-shaped
structure called a Neural Keel.
– The neural keel will push down into the body above the
notochord.
– The neural keel will then detach from the surface and cellular
rearrangements will allow for a neurocoelto form. This gives it a
structure similar to that in the typical vertebrate CNS.

Mesenchyme
• Mesenchyme is an undifferentiated embryonic
tissue.
– It is composed of totipotent cells called Mesenchymal
Cells residing in an amorphous extracellular matrix.
– Mesenchymal cells can develop into any other cell
type.
• They have branching, dendritic processes.
• Mesenchyme makes up much of the early embryonic body.
• Most mesenchyme is of mesodermal origin.

Mesenchyme
– Some, called Mesectoderm, is of ectodermal
origin.
– During organogenesis mesenchyme will become
organized into clusters called Blastemas.
• These blastemas will gradually assume the basic outline
of future organs and their mesenchymal cells will
differentiate to from the appropriate tissues.
– Some Mesenchymal cells are present in the adult.
• They will be pleuripotent in nature allowing them to
replacedamaged/lost cells so as to repair tissues.

The Fate of the Ectoderm
• Ectoderm will give rise to:
– CNS,PNS, and some of the associated membranes,
– sensory epithelium of the special senses and the
retina of the eye,
– epidermis, epidermal derivatives, integumentary
glands,
– the lining of the stomodeum and the proctodeum
and their derivatives,
– numerous other tissues especially those of the
head and pharynx.

Stomodeum and Proctodeum
• The oral cavity begins to form as a midventral
invagination of the ectoderm at the anterior end of the
foregut on the embryonic head.
– This invagination is called the Stomodeum.
• The foregut and stomodeum are initially separated by a
thin partition called the Oral Plate.
– The oral plate will rupture to allow for communication
between the digestive tract and the external environment.
• The stomodeum ectoderm lines the anterior portion of
the oral cavity including at least part of the tongue.
– Some of this stomodeal ectoderm will be invaded by
endoderm making it hard to distinguish which cells
produced which of the glands located at he junction of
ectoderm and endoderm.

Stomodeum and Proctodeum
– In mammals stomodealectoderm gives rise to cells
that secrete tooth enamel.
• In other vertebrates neural crest cells secrete enamel.
– An evagination called Rathke`s Pouchforms from the
roof of the stomodeum prior to the rupturing of the
oral plate.
• Rathke`s pouch will give rise to the adenohypophysis.
• The proctodeum is another ectodermal invagination (like the
stomodeum) but it is associated with the hindgut.
• The proctodeum and hindgut are separated by a thin
partition called the Cloacal Plate temporarily.

The Neural Crest
• When the neural tube formed some of the
neighboring ectodermseparated from the
surface ectoderm and the neural tube
becoming theNeural Crest.
– The neural crest first appears as neuroectodermal
clusters.
• One pair ofneural crest will occur in each metamere
and several clusters will occur lateral to the hindbrain
and midbrain.
• Neural crest cells proliferate rapidly and give rise to a
great number of structures.

The Neural Crest
• When the neural tube formed some of the
neighboring ectoderm separated from the
surface ectoderm and the neural tube
becoming the Neural Crest.
– The neural crest first appears as neuroectodermal
clusters.
• One pair of neural crest will occur in each metamere
and several clusters will occur lateral to the hindbrain
and midbrain.
• Neural crest cells proliferate rapidly and give rise to a
great number of structures

The Neural Crest
• Some cells in the head form blastemas for the skeleton of
the arches including the jaws.
• Some form portions of the neurocranium and some of the
membrane bones that will surround the neurocranium.
• Some are involved in the development of the endocrine
glands (The thyroid, parathyroids, and thymus) and glands
derived from the pharyngeal/branchial pouches called
Ultimobranchial Bodies.
• In agnathans, neural crest cells will give rise to the branchial
basket and lingual cartilages.
• Neural crest mesenchyme in the head or the trunk moves
ventrally around the notochord to form the autonomic
ganglia and adrenal medulla.

The Neural Crest
• Neural crest cells around the neural tube produce
the meninges (excepting the dura mater).
• Some neural crest cells will become a variety of
pigment cells including those of the skin.
• Some neural crest cells remain close to their
point of origin to become the perikarya of
sensory neurons whose perikarya are located in
PNS ganglia.
• Other neural crest cells will migrates distally
along the course of the PNS to give rise to the
neurilemma.

Ectodermal Placodes
• Ectodermal placodes are paired thickenings of localized portions of
theectoderm that sink beneath the skin and will give rise to neurectoderm
and the lens of the eye.
– The resulting neuroectoderm will produce neuroblasts and certain
sensory epithelia.
• Ectodermal placodes can be grouped a:
– one pair ofOlfactory Placodes/Nasal Placodes
• They form above the stomodeum.
• They will become part of the lining of the nasal pits (that open into the nostrils).
• Some of these cells will develop into the neurosensory cells of theolfactory
epithelium.
– one pair of Otic Placodes in the ectoderm lateral to the hindbrain
• The ectoderm of the oticplacodes will become theOtocystsas theysink into the head.
– The otocysts will give rise to the sensory epithelium of the internal ear.

Ectodermal Placodes
– one pair of Optic Placodes/Lens Placodes which will develop
into the lens of the eye.
– a series of Epibranchial Placodes on the side of the head
that will sink into the head along the midbrain and
hindbrain.
• These placodes will contribute neuroblasts that become cellbodies
in one pair or more of the ganglia of CN 7, 9, and 10.
• Their nerve fibers will innervate the taste buds.
– in fishes and amphibians a series of Linear Series of
Placodes will extend the length of he trunk and tail.
• Some of these placodes will migrate into the head.
• They will sink into the body to become the Neuromast Organs of the
cephalic and lateral lines.
– Other placodes on the heads of fishes become
electroreceptive epithelium.

The Fate of the Endoderm
• Endoderm will give rise to the epithelium of the entire digestive tube and
its evaginations.
• Endoderm will also give rise to the epithelia of the parathyroids,
ultimobranchial glands, thymus, lining of eustachian tubes, and the lining
of the middle ear.
– This endoderm derives from the pharyngeal pouches.
• Midventralevaginations of the pharynx such as the thyroid, lungs, and
swim bladders, when present, are lined by endoderm.
• Caudal to the pharynx the endoderm will evaginate to give rise to the
liver, gall bladder, pancreas, crop sacs, and a variety of gastric and
intestinal ceca.
• Since they are derived from the cloaca, mammalian urinary bladders and
urogenital sinuses are typically lined by endoderm.

The Fate of the Mesoderm
• Dorsal Mesoderm or the Epimere
– Most of the dorsal mesoderm will develop into block - like
segments running in pairs lateral to the notochord called Somites.
• They run along the notochord and neural tube throughout the
tail, trunk, and, to a lesser degree, the head.
• The somite will differentiate into three regions (running
superficial to deep):
– Dermatome – gives rise to the dermis but only of the
dorsal body.
» The remainder of the dermis arises from the lateral
platemesoderm.
– Myotome – gives rise to skeletal muscles.
– Sclerotome – gives rise to the vertebrae, ribs, and portions
of the neurocranium.

The Fate of the Mesoderm
–Some of the dorsal mesoderm located
anterior to the somites arearranged into
incomplete segments called Somitomeres.
• Somitomeres lack the dermatome and
sclerotome components.
– So somitomeres give rise to skeletal muscles.
» These muscles are the extrinsic ocular muscles
and thebranchniomeric muscles

Lateral Plate Mesoderm or Hypomere
• Lateral plate mesoderm is confined to the trunk
and divides into somatic mesoderm and
splanchnic mesoderm (which is separated by the
coelom).
• Somatic mesoderm will give rise to:
– connective tissues of the body wall
– blood vessels of the body wall
– the skeleton of the body wall, limb girdles, and limbs
– dermis of the body wall
– and the parietal peritoneum.

Lateral Plate Mesoderm or Hypomere
• Splanchnic mesoderm gives rise to:
– smooth muscles and connective tissue system of the
digestive tube and its outpocketings
– smooth muscles and connective tissue system of the
heart and visceral blood vessels
– The Intermediate Mesoderm or Mesomere
• Intermediate mesoderm is a pair of unsegmented
mesodermal ribbons running the length of the trunk
between the lateral plate mesoderm and the somites.
• It will give rise to kidney tubules and the longitudinal ducts
of the urogenital system.
– For this reason it is also known as “Nephrogenic Mesoderm”.

The Extraembryonic Membranes
• Most vertebrates possess specialized
membranes that extend beyond the
embryonic body.
– They arise early during development and provide
a number of essentialfunctions.
– The chief extraembryonic membranes are: yolk
sac, amnion, chorion, and allantois.
• The yolk sac is the most primitive and is founding all
vertebrates.
• The other three are found only in the amniotes
(reptiles, birds, andmammals).

The Yolk Sac
The yolk sac surrounds the yolk and empties into
the midgut.
– It is lined by endoderm.
– It is highly vascularized.
• The associated blood vessels are termedVitellinearteries,
veins.
• Vitelline vessels are confluent with blood vessels of the
embryonic body.
– As the embryo grows the yolk sac will diminish.
• As it atrophies, the yolk sac will disappear into the ventral
body wall.
– The remaining intracoelomic yolk sac will either
becomeincorporated into the wall of the midgut or remain as an
outpocketing.

The Yolk Sac
• Embryonic sharks develop a large yolk diverticulum (or
outpocketing) within the coelom.
– This yolk diverticulum is still present when the shark is born
and will sustain it for several days.
• In viviparous species of fish and amphibians the yolk
sac, being highly vascularized and close to internal
tissues, will often serve as an oxygen absorbing
membrane to receive oxygen from the mother.
– In this case it is sometimes referred to as a “Simple Yolk Sac
Placenta”.
• Therian mammals lack yolk but still will have a yolk sac.
– It is called MeckelʼsDiverticulum.
– It is a sign of our reptilian heritage.

The Amnion and Chorion
• Amniotic embryos develop within two membranous sacs,
the amniotic and chorionic sacs.
• Both the amnion and chorion form when upfoldings of the
somatopleure meet above the embryo and fuse together.
– This will form the amniotic sac that will surround the embryo.
– The somatopleure continues to extend and will grow around
both the amniotic sac and the yolk sac to give rise to the
chorionic sac.
• The evolution of these membranes allowed for the
development of the amniotic egg.
– The amniotic sac surrounds the embryo and holds a fluid called
Amniotic Fluid.
• Amniotic fluid is mostly metabolic water from embryonic tissues.
• It will protect the embryo from dehydration and mechanical trauma.

The Amnion and Chorion
• The chorion will lay in close association with
either the eggshell or the maternal body wall.
– The chorion will keep the embryo supplied with
oxygen either through the eggshell or from the
mother.
– In viviparous species the chorion also supplies the
embryo with nourishment from the mother.

The Allantois
• The allantois arises as a midventral evagination of the
embryo’s cloaca.
– Normally it grows until it comes in contact with the
chorion.
– This forms the Chorioallantoic Membrane.
• In monotremes and most reptiles the chorioallantoic membrane
serves for respiration.
• In eutherian mammals it is in contact with the maternal lining and
makes up the embryonic portion of the placenta called the
Chorioallantoic Placenta.
– The base of the placenta, that portion closest to the
cloaca, becomes the urinary bladder in amniotes.
– In mammals a portion forms a portion of the umbilicus
called the Urachus or Middle Umbilical Ligament.

The Placenta
• The placenta, in its broadest sense, is any region in a
viviparous animal where maternal and embryonic
tissues of any type are in intimate contact and allow for
an exchange of materials.
– This would include the simple yolk sac placenta of some
non - amnioticspecies.
– Most people consider a true placenta to exist only in
amniotes.
• A true placenta would consist of:
– a highly vascularized region of extraembryonic membrane
suchas yolk sac, chorionic membrane, chorioallantoic
membrane, orchorion
– associated highly vascularized maternal lining

The Placenta
• Eutherian mammals have a chorioallantoic membrane.
• In most marsupials the yolk sac, not the allantois, lays
against the chorion forming a Choriovitelline Placenta.
– The extraembryonic membrane only lays closely against
the endometrium and can simply peel away from the
endometrium during parturition.
– This is called a Contact or Nondeciduous Placenta.
• Placental mammals have finger-like extensions called
Chorionic Villi that invade into the endometrium.
– As a result during parturition a portion of the
endometrium islost.
– This is termed a Deciduous Placenta.

ZooCVABioIII.2013.Abante

Recomendados

Recomendados

Más contenido relacionado

La actualidad más candente

La actualidad más candente (20)

Destacado

Destacado (20)

Similar a ZooCVABioIII.2013.Abante

Similar a ZooCVABioIII.2013.Abante (20)

Más de Mariel Marjes

Más de Mariel Marjes (20)

Último

Último (20)

ZooCVABioIII.2013.Abante