IMPKEGNATION. 



843 



During this period other changes have also taken place. The cephalic 

 and caudal extremities have become fixed and form the cephalic and caudal 

 flexures; and the embryo also being curved upon itself laterally, the vitelline 

 mass appears separated from it by a constriction. This constriction gradu- 

 ally increasing, finally separates the vitelline mass as a vesicular body, it 

 being connected with the body of the embryo by the vitelline duct. (Fig. 

 220.) The vesicular body thus formed is called the umbilical vesicle. This 

 at first communicates with the intestinal cavity, but as development proceeds 

 the duct of communication becomes closed, and the vesicle is merely attached 

 by a pedicle, and finally disappears altogether. At the time of the develop- 

 ment of the bloodvessels, vessels appear on the surface of the umbilical 

 vesicle, constituting the vascular area, the chief vessels being the omphalo- 

 mesenteric arteries and veins. The vessels of the vascular area absorb the 

 nutritive material contained within the vesicle and convey it to the embryo 

 for its sustenance. 



FIG. 220. 



Diagrammatic Section showing the Relation in a Mammal and in a Man between the Primi- 

 tive Alimentary Canal and the Membranes of the Ovum. The stage represented in this diagram 

 corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the ex- 

 pansion of the allantois ; c, the villpus chorion ; a, the amnion ; a', the place of convergence of 

 the amnion and reflection of the false amnion, a" a", or outer or corneous layer ; e, the head and 

 trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal axis ; i, i, the simple 

 alimentary canal in its upper and lower portions ; the yolk-sac or umbilical vesicle ; vi, the vitel- 

 line duct : u, the allantois connected by a pedicle with the anal portion of the alimentary canal. 



786. Shortly after the occurrence of the commencement of the forma- 

 tion of the umbilical vesicle, double folds, formed of the external layer of 

 the blastoderm, are given off from the cephalic and caudal extremities, and 

 laterally, which curve around over the dorsal surface of the embryo, where 

 they meet and coalesce, and, their point of junction becoming absorbed, form 

 the amniotic cavity. (Figs. 220, 221, 222, and 223.) The outer layer of 

 the fold, or false amnion, gradually expands and covers the whole of the in- 

 ternal surface of the vitelline membrane, which it ultimately replaces ; the 



