346 REPRODUCTION 



through the vitelline duct are the future alimentary canal 

 and the yolk-sac, or umbilical vesicle. It is to be noticed 

 that the visceral plates embrace both somatopleure and 



FIG. 101. Diagrammatic section showing the relation in a mammal 



between the primitive alimentary canal and 



the membrane 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 villqus chorion; a, the amnion; a', the place of convergence of the amnion 

 and reflexion of the false amnion; a", a", 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. Immedi- 

 ately beneath the right hand * is seen the fetal heart, lying in the anterior part 

 of the pleuroperitoneal cavity; v, the yolk-sac or umbilical vesicle; vi, the vitello- 

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

 tion of the alimentary canal. (Kirkes after Quain.) 



splanchnopleure, and that it is the ectodermic layers of the 

 splanchnopleure which finally join to form the gut tract, and 

 the somatopleure which forms the ventral ancj lateral walls 



