2. ALIMENTARY SYSTEM II GASTROINTESTINAL TRACT

4. Profile view of a human embryo estimated at twenty or twenty-one days old. (Dorsal aorta labeled at center left.)

37. TIMETABLE OF SOME EVENTS DESCRIBED ABOVE Age Developmental events 16 days Allontoic diverticulum starts appearing 3 weeks Gut begins to acquire tubular form because of head and tail foldings. At the end of 3 rd week the buccopharyngeal membrane ruptures. 4 weeks The fusiform shape of the stomach becomes visible. 5 weeks Stomach rotates and dilates. Intestinal loop begins to form. Caecal bud can be identified. 6 weeks Intestinal loop is well formed. Urorectal septum starts dividing the cloaca. Allantois and appendix become clearly visible. Stomach complete its rotation. 7 weeks Septation of cloaca into rectum and urogenital sinus is completed. Intestinal loop herniates out of the abdominal cavity. 8 weeks Intestinal loop rotates counterclockwise. 9 weeks Anal membrane breaks down. 3 months Head and tail foldings are completed. Herniated coils of intestine return to the abdominal cavity.