TROPHOBLAST AND SEROSA. 593 



insect egg with the embryonic tract lying upon the yolk (Figs. 

 1, A and b). 



At a later stage the trophic vesicle develops a caudal exten- 

 sion, so_^[that the embryo attains a more central position, 

 although the cephalic portion of the vesicle is, as a rule, con- 

 siderably larger than the posterior portion (Fig. 2, a). 



A 1^ 



Fig. 1. — A. Embryo of Peripatus novse-britannise, with embryonic 

 tract at posterior ventral extremity of trophic or blastodermic vesicle 

 (original). 



B. Egg of Gryllus, with embryonic tract at the posterior ventral 

 surface of the vitellus. (After Heymous.) 



The young embryo rests upon the trophic vesicle as on a 

 cushion, and there can be little doubt that in the fresh condi- 

 tion, when the cavity of the vessel is filled to repletion with its 

 nutrient fluid contents, the embryo lies in a sort of lap or bay 

 or depression, bounded by turgid lips, produced by the inflation 

 of the surrounding thin wall of the vesicle. I cannot state 

 this as a definite fact as I did not observe the living embryos. 

 Everything, however, points to such a state of things. 



