THE UMBILICAL VESICLE. 



747 



(fig. 212). From that portion of the mucous layer which 

 is now enclosed within the body of the embryo, the intes- 

 tinal canal is developed. 



Fig. 212.* 



Thus, by the constriction which the fold of germinal 

 membrane, in which the abdominal walls are formed, pro- 

 duces at the umbilicus, the body of the embryo becomes 



* Fig. 212. Diagrammatic section showing the relation in a mammal 

 and in man hetween 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 emhryo, previous 

 to the expansion of the allantois : c, the villous choriea ; 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 ; v, the yolk- 

 sac or umbilical vesicle ; v i, the vitello-intestinal opening ; , the 

 allantois connected by a pedicle with the anal portion of the alimentary 

 canal. 



