[Illustration: FIG. 100.--Schema of human embryonic intestinal canal after differentiation of the large and small intestine.]
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).
[Illustration: 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.)]
[Illustration: FIG. 102.--Representation of alimentary canal and appendages of human embryo of 4.1 mm.; isolated. × 15. (Kollmann, after His.)]
[Illustration: FIG. 103.--Alimentary canal and appendages of human embryo of 12.5 mm. × 12. (Kollmann, after His.)]
[Illustration: 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.]
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.
The primitive condition of the embryonal mammalian alimentary tract, after differentiation of the large intestine is well illustrated by some of the lower vertebrates in which development never proceeds beyond this stage. Fig. 112 shows the entire alimentary canal of a teleost fish, the conger eel (_Echelus conger_) isolated.