584 



GENERATION AND DEVELOPMENT. 



that contains it, are separated into two portions, one of which 

 is retained within the body of the embryo, while the other re- 

 mains outside, and receives the name of the umbilical vesicle 

 (v, Fig. 219). The cavity of the latter communicates for some 

 time with that of the abdomen, through what is called the 

 umbilicus, by means of a gradually narrowing canal, called 

 the vitelline duct; the interior of the abdomen and that of the 



Diagrammatic section showing the relation in a mammal and in man between the 

 primitive 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 expansion of the allantois : c, the villous chorion ; a, the 

 amnion ; a', the place of convergence of the amnion and reflection of the false am- 

 nion a" a", or outer or corneous layer; e, the head and trunk of the embryo, com- 

 prising the primitive vertebrae and cerebro^spinal axis ; i, i, the simple alimentary 

 canal in its upper and lower portions; v, the yelk-sac or umbilical vesicle ; v i, the 

 vitello- intestinal opening; u, the allantois connected by a pedicle with the anal por- 

 tion of the alimentary canal. 



umbilical vesicle being lined by a continuous layer of the 

 inner stratum, or mucous layer of the germinal membrane ; 

 while around both of them is a continuation of the outer, or 

 serous layer (Fig. 219). From that portion of the mucous layer 



