416 VERA DANCHAKOFF 



and invariably retain their reciprocal connections. Groups of 

 epithelial cells are however found separated from the larger 

 accumulations and here often develop into typical pearls. 



The cornification of the embryonic cubic cells after a activa- 

 tion which transformed the thin subvascular layer into a strati- 

 fied epithelium may serve as a conclusive proof of their ecto- 

 dermal origin. The superficial localization of the respiratory 

 capillary net is therefore not the passive result of a supposed de- 

 generation of the ectodermal cells above it, but it is due to an 

 active growth of the vessels into and through the ectoderm. 

 The latter is found secondarily lying beneath the vascular net 

 normally in the form of a double layer of cubic cells. 



The study of the allantois has shown an example of a peculiar 

 rearrangement of the derivatives of the germlayers; tissues of 

 mesodermic origin appear on the surface of the allantois and an 

 ectodermal epithelial membrane is thereby pushed deeper into 

 the mesenchymal layer of the allantois. Bpth structures re- 

 tain in their new position their specific characters. More- 

 over the ectodermal membrane, the cells of which continue to 

 proliferate under the new environment given by experiment, 

 develop and differentiate in a definite manner elsewhere 

 exclusively characteristic of epithelial ectodermal tissue. This 

 may suggest, that the ectoderm of the serosa has reached a stage 

 at which the cells have acquired a specificity sufficient to make 

 them differentiate along definite lines under any environmental 

 conditions. 



