694 



ARTHUR WILLEY. 



As the embryo advances iu development, the relative dimen- 

 sions of the trophic organ decrease pari passu with the 

 growth of the embryo. The posterior portion of the vesicle is 

 the first to disappear, being used up in the formation of the 

 dorsal body-wall of the embryo. When there is no longer any 

 caudal extension of the vesicle there may still be observed a 



Fig. 2. — a. Embryo of Peripatus novse-br itannise, showing posterior 

 extension of trophic vesicle and primary ventral flexure of embryonic 

 tract. (Original.) 



B. Egg of S trongylosoma guerinii, Gerv. (a Diplopod), showing 

 primary ventral flexure of embryonic tract. (After Metschnikofi".) 



large head-fold, which, in consequence of the cephalic flexure 

 subsequently undergone by the embryo, is reflected like a cap 

 over the ventral surface of the embryo (14, pi. iii, fig. 35). 



