. A text-book of embryology for students of medicine [electronic resource]. Embryology; Embryology. 190 TEXT-BOOK OF EMBRYOLOGY. spending walls of the body-cavity become drawn out so as to constitute in each case a median vertical fold consisting of two closely approximated layers of serous membrane with a little connective tissue between them. These folds are the dorsal and the ventral mesenteries (Fig. 93). While the. A G Fig. 93.—Diagrammatic cross-sections of the body of the embryo in the region of the heart at level of future diaphragm : a, esophageal segment of gut-tract; b, dorsal mesen
. A text-book of embryology for students of medicine [electronic resource]. Embryology; Embryology. 190 TEXT-BOOK OF EMBRYOLOGY. spending walls of the body-cavity become drawn out so as to constitute in each case a median vertical fold consisting of two closely approximated layers of serous membrane with a little connective tissue between them. These folds are the dorsal and the ventral mesenteries (Fig. 93). While the. A G Fig. 93.—Diagrammatic cross-sections of the body of the embryo in the region of the heart at level of future diaphragm : a, esophageal segment of gut-tract; b, dorsal mesentery; c, mesocardium posterius; d, mesocardium anterius; e, begin- ning of septum transversum, containing vitelline and allantoic veins ; /, septum transversum ; g, thoracic prolongation of abdominal cavity; nc, neural canal. dorsal mesentery extends throughout the entire length of the canal, the ventral fold is present only at its anterior or upper part, corresponding in the extent of its attachment to the digestive tube to that portion representing the future stomach and upper part of the duodenum (Fig. 94). The ventral mesentery at first is present throughout the entire extent of the canal, but very early undergoes obliteration except in the situation above noted. Concerning the reason and the method of its disappearance nothing is definitely known. The intestinal tube, at a comparatively early stage, pre- sents on its ventral surface near the posterior or caudal end a small evagination that enlarges to form the allantois (see p. 89). While a part of the intra-embryonic portion of the allantois dilates and develops into the bladder, the part be- tween this latter and the intestine is known as the urogenital sinus. The part of the gut-tube posterior to, caudad of, the origin of the allantois, is a blind pouch known as the cloaca. The latter is, therefore, the common termination of the urinary and the intestinal tracts. To repeat, we have now, in the third week of developmen
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