[Illustration: FIG. 42.--Genito-urinary tract and cloaca of _Iguana tuberculata_, female. (Columbia University Museum, No. 1846.)]
After the intestinal canal has become separated from the yolk-sac it forms at first a straight tube, running cephalo-caudad beneath the chorda dorsalis. In most forms, however, the intestine grows much more rapidly in length than the body-cavity of the embryo in which it is contained. Hence the intestine is forced to form coils or convolutions.
The entire alimentary canal, from the mouth to the anus, can be separated into the following divisions and subdivisions:
1. The oral cavity. 2. The pharynx. 3. The œsophagus. 4. The stomach.
=II. Midgut=, closely associated at its beginning with the liver and pancreas.
It extends between the pyloric extremity of the stomach and the beginning of the last segment, the endgut, frequently separated from both by ring-like aggregations of the circular muscular fibers and corresponding projections of the mucous membrane (pyloric and ileo-colic valves).
The midgut is usually the longest portion of the intestinal tube.
=III. Endgut=, the last segment of the intestinal canal, courses through the pelvic portion of the body cavity. From this short end-piece are developed: (1) The colon, sigmoid flexure and rectum; (2) the cloaca with the uro-genital sinus and the duct of the allantois.
ANATOMY OF THE PERITONEUM AND ABDOMINAL CAVITY.
For the purpose of studying the adult human peritoneum it is in the first place absolutely necessary to obtain a correct appreciation of the disposition of the chief viscera within the abdominal cavity and of their mutual relations. In the second place the visceral vascular supply of the abdomen must be carefully considered in order to correctly appreciate certain important relations of the peritoneal membrane.
A review of the visceral contents of the abdomen shows that we have to deal chiefly with the divisions of the alimentary tract below the œsophagus and the structures directly derived from the same, as liver and pancreas, or associated topographically with the alimentary canal, as the spleen. Portions of the urinary and reproductive systems situated within the abdominal and pelvic cavities will also require consideration.