Fig. 97.—Alimentary canal of human embryo of 5 mm. × 15. (Reconstruction after His.)
Fig. 98.—Schema of human embryonic intestinal canal, with intestinal umbilical loop, but before differentiation of the large and small intestine.
Fig. 99.—Viscera of Necturus maculatus, mud-puppy, in situ. (Columbia University Museum, No. 1175.)
1. In embryos up to 5 mm. cervico-coccygeal measure (Fig. 97) the intestinal tube follows the body curve without deviation. Subsequently the elongation of the intestine causes a small bend, with the convexity directed ventrad, to appear in the umbilical region. This bend gradually increases until the gut forms a single long loop, beginning a short distance below the pylorus and directed ventro-caudad. The apex of the loop, to which the vitello-intestinal duct is attached (Fig. 98) (cf. [p. 34]) projects beyond the abdominal cavity into the hollow of the umbilical cord, constituting the so-called “umbilical or embryonal intestinal hernia.” This entrance of the apex of the intestinal umbilical loop into the umbilical cord begins in embryos of about 10 mm. During the succeeding weeks—up to the tenth—the segment of the intestine thus lodged within the hollow of the umbilical cord increases. After this period the intestinal coils are gradually withdrawn within the abdomen. The explanation of this temporary extrusion of the intestine into the umbilical cord is probably to be found in the strain produced by the yolk-sac which is attached by the vitello-intestinal duct to the apex of the umbilical loop. As we have seen ([p. 35]) the site of the original apex of the loop may still be indicated in the adult by the persistence of a portion of the vitello-intestinal duct as a “Meckel’s diverticulum.”
In its simplest primitive condition the loop presents a proximal, descending or efferent limb, an apex, and an ascending, returning or afferent limb (Fig. 98). In the human embryo these segments of the loop furnish the jejuno-ileum and portions of the large intestine, in a manner to be subsequently detailed.
This stage in the development of the higher vertebrate intestine is well illustrated by the alimentary tract of the mud-puppy, Necturus maculatus, shown in [Fig. 99], which represents the entire situs viscerum of an adult female animal.
The stomach is tubular, not distinctly differentiated from the œsophagus, placed vertically in the long axis of the body. The pyloric end is marked by a constriction separating stomach from midgut and immediately beyond this point the pancreas is applied to the intestine. The rest of the intestinal canal forms a simple loop, the descending limb presenting one or two primitive convolutions. There is no marked differentiation between large and small intestine, the canal possessing a nearly uniform caliber from pylorus to cloaca.