2. The differentiation of the small from the large intestine, marked by the appearance of the cæcal bud or protrusion ([Fig. 100]), takes place in the ascending segment of the umbilical loop a short distance from the apex. In the human embryo the cæcal bud appears in the 6th week as a plainly marked protuberance, which grows very slowly in length and circumference. It shows very early an unequal rate of development; the terminal piece, not keeping pace in growth with the proximal portion, is converted into the vermiform appendix, while the proximal segment develops into the cæcum proper. The increase in the length of the loop, which begins to be marked in the 7th week, is not uniform. The apex is the first portion to present the evidences of this growth. Subsequently the descending limb grows in length very rapidly and is early thrown into numerous coils of the future mobile portion of the small intestine (jejuno-ileum). Even before the withdrawal of the apex of the loop within the abdominal cavity a prominent coil of these convolutions is found protruding in the umbilical region ([Fig. 544]). The ascending limb of the loop from which a portion of the large intestine is developed, grows comparatively slowly at this time.

Fig. 100.—Schema of human embryonic intestinal canal after differentiation of the large and small intestine.   Fig. 101.—Human embryo of 2.15 mm., twelve days old. Seessel’s sac is the cephalic blind termination of the embryonic foregut before the communication with the ectodermal invagination of the stomadæum has been formed. (Reconstruction after His.)
Fig. 102.—Representation of alimentary canal and appendages of human embryo of 4.1 mm.; isolated. × 15. (Kollmann, after His.)   Fig. 103.—Alimentary canal and appendages of human embryo of 12.5 mm. × 12. (Kollmann, after His.)

Fig. 104.—A. Schematic representation of alimentary canal, with umbilical loop and mesenteric attachments in human embryo of about six weeks. B and C, stages in the intestinal rotation.

The future portions of the human adult alimentary tract below the stomach may be referred, in reference to their derivation, to this primitive condition of the tube as follows:

1. The segment of small intestine situated between the pylorus and the beginning or point of departure of the proximal or descending limb of the umbilical loop, develops into the duodenum. This portion of the small intestine is indicated early in embryos of 2.15 mm. (Fig. 101), by the origin of the hepatic duct from the intestinal tube. Somewhat later, in embryos of 4.10-5 mm. length, (Fig. 102) it becomes additionally marked by the origin of the pancreatic diverticulum. The duodenum, at first straight, now begins to curve, forming a short duodenal loop or bend. In embryos of 6 weeks the duodenum forms a simple loop placed transversely below the pyloric extremity of the stomach (Figs. 103 and 104).

2. The descending limb, the apex and a small part of the ascending limb of the umbilical loop form the jejuno-ileum.

3. The remainder of the ascending limb forms the cæcum and appendix, the ascending and transverse colon.

4. The distal straight portion of the primitive tube forms the terminal portion of the transverse colon (the splenic flexure), the descending colon, sigmoid flexure and rectum.