28 



PRELIMINARY ACCOUNT. 



Fig. 9. 



[chap. 



vt 



D 



-H* 



Fig. 9, A to N forms a series of purely diagrammatic repre- 

 sentations introduced to facilitate the comprehension of the 

 manner in which the body of the embryo is formed, and of the 

 various relations of the yolk-sac, amnion and allantois. 



In all vt is the vitelline membrane, placed, for convenience 

 sake, at some distance from its contents, and represented as per- 

 sisting in the later stages ; in the actual egg it is in direct contact 

 with the blastoderm (or yolk), and early ceases to have a separate 

 existence. In all e indicates the embryo, pp the general pleuro- 

 peritoneal space, of the folds of the amnion proper ; ae or ac the 

 cavity holding the liquor amnii ; al the allantois ; a the ali- 

 mentary canal ; y ov ys the yolk or yolk-sac. 



A, which maybe considered as a vertical section taken longi- 

 tudinally along the axis of the embryo, represents the relations of 

 the parts of the egg at the time of the first appearance of the 

 head-fold, seen on the right-hand side of the blastoderm e. The 



