TROPHOBLAST AND SEROSA. 



595 



This head- fold or cephalic portion of the trophic 

 vesicle is attached to the embryo in the nuchal region; 

 in other words, the point at which the final resorption of the 

 trophic organ of these embryos takes place, lies in the nuchal 

 region (cf. Fig. 3, a). This fact is of capital importance when 

 taken in comparison with the remarkable phenomena attendant 

 upon the involution of the serosa of insect embryos. 



ser--, 



Fig. 3. — a. Embryo of Peripatus novse-britan nise, in which the posterior 

 extension of trophic vesicle has almost entirely disappeared. The ante- 

 rior portion of the vesicle is present as a large lobe attached to the 

 head. The figure also illustrates the caudal flexure in addition to the 

 ventral flexure of the embryo. (After Willey, ' Zoological Results,' 

 Cambridge, 1898.) 



B. Egg of Gryllus, at stage after the eversiou of the embryo, am. 

 Amnion reflected over dorsum of embryo, ser. Serosa withdrawn to 

 anterior portion of vitellus and attached to head of embryo, like the 

 trophic vesicle in a. A slight caudal flexure is present, but no ventral 

 flexure. (After Heymons.) 



After the amnion of the Insecta Pterygota is formed by 

 the fusion of tail-fold, lateral folds, and head-folds, the amniotic 



