1158 



EMBRYOLOGY. 



remarkably curved on itself (cephalic flexure), and a smaller but similar folding- 

 over takes place at its hinder end (caudal flexure). At the same time the sides of 

 the embryo, formed by the somatopleure, grow and curve ventrally toward each 

 other, so that the embryo at this stage is aptly compared to a canoe turned over, 

 and becomes marked off' from the general blastoderm by a limiting sulcus. In 

 consequence of this incurving of the embryo, both in an antero-posterior and a 

 lateral direction, the blastodermic vesicle becomes nipped by the somatopleure and 

 resembles an hour-glass with two unequal parts. The smaller portion is enclosed 

 within the body of the embryo, and constitutes the enteron or primitive alimentary 

 canal, while the larger portion, left outside the embryo, is termed the yolk-sac or 

 umbilical vesicle. These two parts of the original blastodermic vesicle communicate 

 through the constricted portion, which is the site of the future umbilicus, and, 



FIG. 703. Diagrammatic action through the ovum of a mammal in the long axis of the embryo, e. The 

 cranio-vertebral axis, i, i. The cephalic and caudal portions of the primitive alimentary canal, a. The amnion. 

 a'. The point of reflection into the false amnion. v. Yolk-sac, communicating with the middle part of the 

 intestine by v i, the vitello-intestinal duct. u. The allantois. The ovum is surrounded externally by the vil- 

 lous chorion. 



when the body cavity is ultimately closed at the umbilicus, the constriction is 

 narrowed to form a small duct, the omphalo-mesenteric or vitelline duct (Figs. 702, 

 703, and 705). The cephalic part of the primitive alimentary canal is named the 

 fore-gut, the caudal portion the hind-gut, while the intermediate portion which 

 communicates directly with the yolk-sac, is termed the mid-gut. The yolk-sac is 

 of small importance and very temporary duration in the human subject. It is for 

 the purpose of supplying nutrition to the embryo during the very earliest period 

 of its existence. In the oviparous animals, however, where no supply of nourish- 

 ment can be obtained from the mother, since the egg is entirely separated from 

 her, the yolk-sac is large and of great importance, as it supplies nutrition to the 

 chick during the whole of foetation. Vessels developed in the mesoblast soon 

 cover the yolk-sac, forming the vascular area ; these are named the omphalo- 

 mesenteric vessels, and are two in number (Fig. 704). They appear to absorb the 

 fluid of the yolk-sac which, when the fluid has disappeared, dries up and has no 

 further function. The activity of the yolk-sac ceases about the fifth or sixth week, 

 at the same time that the allantois, which is the great bond of vascular connection 

 between the embryo and the uterine tissues, is formed. The yolk-sac remains 

 visible, however, up to the fourth or fifth month, with its pedicle and the omphalo- 



