BODY CAVITY 345 



ectodermic layers of the splanchnopleure which finally join to 

 form the gut tract, and the somatopleure which forms the ventral 

 and lateral walls of the body cavity. The gut tract has the 



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 seven- 

 teenth 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 re- 

 flexion 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. Immediately beneath the 

 right hand i 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; M, the 

 allantois connected by a pedicle with the hinder portion of the alimentary canal. 

 (Kirkes after Quain,) 



shape of a straight tube occupying the long axis of the embryo 

 and opening into the umbilical vesicle. 



