Dr. Dinesh C. Sharma,
Associate Professor and Head-Zoology
K.M. Govt. Girls Post Graduate College,
Badalpur, G.B. Nagar-U.P., INDIA

Theory of preformation,
Epigenetic theory,
Theory of pengenesis,
Recapitulation theory,
Germplasm theory,
Mosaic theory,
Regulated theory,
Gradient theory
Theory of organizers.

Some Puzzling Things……..
William Harvey concluded that the womb conceives the embryo as the brain
conceives a thought. Harvey believed that coitus merely excites conception in
the uterus as desire is generated in the brain, and the present use of the
words conceive and conception is not without significance in this regard.
Pythagoras regarded semen as a foam of the purest blood and as an excess of
nutriment, and woman also produces semen.
This idea may have had a various origin, but since they knew the spawn of the
female fish and the milt of the male, as well as the sexual discharges of
amphibia, it is not improbable that the superficial similarity between the
testis and the ovary and between the cervical, uterine and tubal secretions
and semen, may have been partly responsible for this idea.
Anaxagoras thought that male individuals arise from sperm produced by the
right and females from that produced by the left testis,
Hippocrates, held that semen is a product of the whole body, He thought that
maleness and femaleness are determined by the excess of male or female
semen present at the time of conception

Aristotle stated “there is evidence that the semen is in the catamenia, for, as
said before, this secretion appears in the male at the same time of life as the
catamenia in the female ” and continues to say that “the spermatic secretions”
are produced b_v “the uterus and pudenda and breasts,” including milk because
it is a nutriment. Aristotle held that the loss of semen is exhausting because
“the body is deprived of the ultimate gain drawn from the nutriment.”
The embryo to Aristotle was “the first mixture of male and female” and the
ovum an oviform body found in the uterus, as it was also to Harvey.
Aristotle says Empedocles thought that sex was determined by the
temperature in the uterus. If hot a male results, if cool a female.

The Theory of Preformation or Preexistence
The preéxistence term was first used by Sir Kenelm Digby in
1644, This preformation idea was also called the theory of
evolution, but according to it organisms were not thought of
as slowly unfolding or evolving, but merely as increasing in
size from a microscopic miniature to the adult.
In the history of biology, preformationism (or preformism) is a
formerly-popular theory that organisms develop from
miniature versions of themselves. Instead of assembly from
parts, preformationists believed that the form of living things
exist, in real terms, prior to their development. It suggests
that all organisms were created at the same time, and that
succeeding generations grow from homunculi, or animalcules,
that have existed since the beginning of creation.
A tiny person inside a
sperm, as drawn by
Nicolaas Hartsoeker in
1695
Joseph of Aromatari, who was enthused over the revelations
of the microscope, and while examining seeds was impressed
by the resemblances of the germ and cotyledons to a plant
and hence announced, in 1625, that all plants were contained
in miniature within the seed.

From painstaking and really very skilled dissections of the larvae
and pupae of flies and of butterflies, Swammerdam also was led
to conclude that all the parts of these adult animals are
contained in miniature in the immature forms. His skill in
dissection and representation was unsurpassed, but he allowed
his imagination -to carry him so far that, according to Boerhaave,
he actually demonstrated all parts of the butterfly in the body of
a caterpillar at a meeting of scientists.
Surely there must have been some doubting Thomases present!
Swammerdam apparently was misled by what he saw in the
pupal stage, and from the presence of all parts concluded that all
organs also exist in the larva and the ovum. This seemed only a
small and logical step from his observations upon dissections
and this Swammerdam took. He wrongly opposed the idea of
metamorphosis, as illustrated in the development of the butterfly
which he studied, but it seems that he was the first to represent
developing frog eggs showing cleavage.

Spallanzani says he announced his discovery of the
preéxistence of the germ in a species of frog, in his
Prospectus concerning animal reproduction, published
in 1768. In the introduction to his dissertations relating
to the “Natural History of Animals and Vegetables,” he
wrote:
“Having examined other animals, and having found that the same thing is true
with respect to them, I have still stronger reason for presuming that the
existence of the germ in the female before fecundation is one of the most
general laws of nature. . . . I have been led by observations, which show the
preexistence of the germ, to discover that an order of animals, considered by
naturalists as oviparous, is in reality viviparous.”
He not only believed that the embryo preéxisted in the ovum. but that the
amnion and umbilical cord also did so even before fertilization, and insisted
that ova, hence, were not such, but fetuses. He held that tadpoles of frogs and
toads were likewise contained in the ova before fertilization while still in the
ovary, saying:

Epigenetic theory
Development before birth, including gametogenesis, embryogenesis, and fetal
development, is the process of body development from the gametes are formed
to eventually combine into a zygote to when the fully developed organism exits
the uterus. Epigenetic processes are vital to fetal development due to the need
to differentiate from a single cell to a variety of cell types that are arranged in
such a way to produce cohesive tissues, organs, and systems.
The term epigenetics (or epigenesis)
referred to the hypothesis that
embryo development occurs as a
progressive and gradual
differentiation of the unstructured
egg. This contrasts with the other
classical hypothesis of preformation,
whereby the embryo was argued to
develop by a process of enlargement
and elaboration of preexisting
structures present within the egg.

Theory of pengenesis,
Charles Darwin's pangenesis theory postulated that every part of
the body emits tiny particles called gemmules which migrate to the
gonads and are transferred to offspring. Gemmules were thought to
develop into their associated body parts as offspring matures. The
theory implied that changes to the body during an organism's life
would be inherited, as proposed in Lamarckism.

Pangenesis mirrored ideas originally formulated by
Hippocrates and other pre-Darwinian scientists, but
built off of new concepts such as cell theory,
explaining cell development as beginning with
gemmules which were specified to be necessary for
the occurrence of new growths in an organism, both
in initial development and regeneration. Darwin wrote
that Hippocrates' pangenesis was "almost identical
with mine—merely a change of terms—and an
application of them to classes of facts necessarily
unknown to the old philosopher
Darwin hypothesized that gemmules might be able to survive and multiply
outside of the body in a letter to J. D. Hooker in 1870. Some gemmules were
thought to remain dormant for generations, whereas others were routinely
expressed by all offspring. Every child was built up from selective expression of
the mixture of the parents and grandparents' gemmules coming from either
side.
Darwin's description of cell proliferation using
pangenesis theory

Hugo de Vries characterized his own version of pangenesis theory in
his 1889 book Intracellular Pangenesis with two propositions, of which
he only accepted the first:
I. In the cells there are numberless particles which differ from each
other, and represent the individual cells, organs, functions and
qualities of the whole individual. These particles are much larger
than the chemical molecules and smaller than the smallest known
organisms; yet they are for the most part comparable to the latter,
because, like them, they can divide and multiply through nutrition
and growth. They are transmitted, during cell-division, to the
daughter-cells: this is the ordinary process of heredity.
II. In addition to this, the cells of the organism, at every stage of
development, throw off such particles, which are conducted to the
germ-cells and transmit to them those characters which the
respective cells may have acquired during development.
De Vries also coined the term 'pangene' which 20 years later was
shortened by Wilhelm Johannsen to gene.

Recapitulation theory,
The theory of recapitulation, also called the
biogenetic law or embryological parallelism—often
expressed using Ernst Haeckel's phrase "ontogeny
recapitulates phylogeny"—is a historical hypothesis
that the development of the embryo of an animal,
from fertilization to gestation or hatching (ontogeny),
goes through stages resembling or representing
successive adult stages in the evolution of the
animal's remote ancestors (phylogeny).
It was formulated in the 1820s by Étienne Serres based on the work of Johann
Friedrich Meckel, after whom it is also known as Meckel–Serres law.
Since embryos also evolve in different ways, the shortcomings of the theory had
been recognized by the early 20th century, and it had been relegated to
"biological mythology"by the mid-20th century

Modern status
Modern evolutionary developmental biology
(evo-devo) follows von Baer, rather than
Darwin, in pointing to active evolution of
embryonic development as a significant
means of changing the morphology of adult
bodies. Two of the key principles of evo-devo,
namely that changes in the timing
(heterochrony) and positioning (heterotopy)
within the body of aspects of embryonic
development would change the shape of a
descendant's body compared to an
ancestor's, were however first formulated by
Haeckel in the 1870s. These elements of his
thinking about development have thus
survived, whereas his theory of
recapitulation has not.
The Haeckelian form of recapitulation theory is considered defunct. Embryos do
undergo a period where their morphology is strongly shaped by their
phylogenetic position, rather than selective pressures, but that means only that
they resemble other embryos at that stage, not ancestral adults as Haeckel had
claimed.

Germplasm theory,
Germ-plasm theory is a concept of the physical basis of
heredity, put forward by August Weismann. According to
his theory, germplasm is the essential element of germ
cells (eggs and sperm) and is passed from one generation
to the other.
His theory states that multicellular organisms consist of germ cells that
contain and transmit heritable information, and somatic cells which carry out
ordinary bodily functions. In the germ plasm theory, inheritance in a
multicellular organism only takes place by means of the germ cells: the
gametes, such as egg cells and sperm cells. Other cells of the body do not
function as agents of heredity. The effect is one-way: germ cells produce
somatic cells, and more germ cells; the germ cells are not affected by anything
the somatic cells learn or any ability the body acquires during its life. Genetic
information cannot pass from soma to germ plasm and on to the next
generation.
This is referred to as the Weismann barrier. This idea, if true, rules out the
inheritance of acquired characteristics as proposed by Lamarck and implied by
Charles Darwin's pangenesis theory of inheritance.

Mosaic theory,
Roux to formulate the Mosaic Theory of development, or the theory that the cell
separated hereditary materials in different amounts to daughter cells at cell division, a
process called qualitative division
Edmund Beecher Wilson experimented with Amphioxus (Branchiostoma)
embryos in 1892 to identify what caused their cells to differentiate into
new types of cells during the process of development. Wilson shook
apart the cells at early stages of embryonic development, and he
observed the development of the isolated cells. He observed that in the
normal development of Amphioxus, all three main types of symmetry, or
cleavage patterns observed in embryos, could be found.
Wilson proposed a hypothesis that reformed the Mosaic Theory
associated with Wilhelm Roux (1893) in Germany. Wilson suggested that
cells differentiated into other cells when influenced by physiological
(dynamic) changes in the hereditary substance contained in cells, and
not because of the qualitative division, or parcelling out, of the
substance into daughter cells.

Gradient theory
It indicates that the rate of metabolism is more in the animal pole than the
vegital pole which is called as metabolic axial gradient. Horstadius &
Runnstrom called it as double gradient thoery. They explain egg development
on the basis of animal pole gradient & vegital pole gradient.
It was given by T. Boveri & was supported by M.C. Child. Child called it as
metabolic axial gradient theory.
According to it, the egg has distinct animal & vegital poles.
The cytoplasm of the animal pole divides repidly but the rate of cleavage in the
vegital pole is slow. It indicates that the rate of metabolism is more in the
animal pole than the vegital pole which is called as metabolic axial gradient.
Horstadius & Runnstrom called it as double gradient thoery. They explain egg
development on the basis of animal pole gradient & vegital pole gradient.

Theory of organizers.
The Spemann-Mangold organizer is a group of cells that are responsible for the
induction of the neural tissues during development in amphibian embryos. First
described in 1924 by Hans Spemann and Hilde Mangold, the introduction of the
organizer provided evidence that the fate of cells can be influenced by factors
from other cell populations. This discovery significantly impacted the world of
developmental biology and fundamentally changed the understanding of early
development.
Hans Spemann showed that transplanting presumptive epidermis into the area of presumptive neural tissue would change
the fate of the transplanted cells to that of their new destination, and likewise when he transplanted presumptive neural
tissue to where the presumptive epidermis was forming. Spemann also showed that by transplanting a piece from the upper
blastopore lip into an area of presumptive epidermis, a secondary embryonic primordium formed, including a secondary
neural tube, notochord and somites. Additionally, splitting the embryo in half and rotating the animal pole in respect to the
vegetal pole resulted in determination spreading from the lower vegetal pole, where the upper blastopore lip was located,
to the upper animal half. He also fused together two identical halves from different embryos and observed formation of the
neural plate. This work provided the initial evidence to support the notion that there existed some “organization center”
that was determined prior to the other embryonic tissue and influenced the determination of surrounding cells

To test this hypothesis, Spemann, along with Hilde Mangold, performed experiments
between 1921 and 1922 using embryos from Triturus cristatus and Triturus taeniatus that
were undergoing gastrulation. The experiment performed resembled the one done in 1918,
however instead of a homoplastic transplantation they used embryos from two species of
newt that are closely related. One of the benefits of using the cristatus and taeniatus
embryos was that the cristatus embryo cells lacked pigment so the fate of the transplant
could be easily tracked when placed among the pigmented taeniatus cells. A piece from the
upper blastopore lip was removed from the cristatus embryo and transplanted into a ventral
region of presumptive epidermis in the taeniatus embryo, away from the developing host
blastopore. Following this transplant, they observed the formation of a secondary
embryonic primordium, consistent with their previous work. This secondary embryo had the
normal features of the primary embryo, including structures such as the neural plate and
notochord, although they lagged slightly in development. Sectioning of the embryo showed
that cells from the transplant were incorporated into the mesoderm, the neural plate, and
constituted almost the entire notochord of the secondary embryo. It was further shown that
the neural plate was almost entirely composed of cells from the host taeniatus embryo.
These experiments concluded that a piece of the upper blastopore lip can be transplanted
into the indifferent tissue of another embryo and induce the host tissue into the formation
of a secondary embryo, therefore implicating the transplanted tissue as an “organization
center”.
The discovery of the Spemann-Mangold Organizer is considered one of the most influential
findings in the field of developmental biology and resulted in Hans Spemann being awarded
the Nobel Prize in 1935 for his work.The mechanisms of how this organizer operates has
been the subject of decades of follow up research.

Regulated theory,