412 VERA DANCHAKOFF 



in the form of a rich plexus. First developed between the germ- 

 layers the vessels are not necessarily confined to this location 

 and seem to be perfectly adapted to an existence within the 

 ectoderm as well as on its free surface. 



Localization of vessels inside of the epithelial membrane is 

 not wholly unknown.^ Even within the trophospongium of a cell 

 capillaries have been observed.'' An expansion of a capillary, 

 particularly of a respiratory net above the surface of the epithe- 

 lium, so far as I am aware, has not been observed. Of course, 

 even in this case, the vessels are not in immediate contact with 

 the air which contains the oxygen; they are covered by the egg 

 shell membrane and by the egg shell, but neither of them are 

 living cellular layers, and the oxygen reaches the endothelial 

 wall of the capillaries without passing through a living substance. 



A study of the gradual development of the allantois has lead 

 me to a different interpretation of the subvascular epithelial-like 

 cells from that given by Ftillborn. This layer of cubic cells, 

 found beneath the capillary net is according to him of mesoder- 

 mal origin, but in reality they are the ectoderm of the serosa. 

 Recent studies tend to show that the morphological structure of 

 the individual cells does not present criteria adequate for their 

 identification, and this is especially true for young undifferen- 

 tiated cells. Even less information concerning the origin of the 

 tissue can be sought from the study of the specific arrangement 

 of cells in tissues; epithelial membranes are formed by ecto- and 

 entoderm as well as by mesoderm; reticular tissue may derive 

 from any germlayer. Inasmuch as the structure of the nuclei 

 and of the cytoplasm of the epithelial cubic cells beneath the 

 vascular net markedly differ from the cells of the endothelial 

 capillary walls, it is permissible to speak of two different struc- 

 tures. The ectodermal origin of the epithelial membrane cannot 

 be however necessarily deduced either from its specific struc- 

 tural features or from the epithelial arrangement of its constitu- 



^ For references see: Traite d'histologie. A. Prenant, P. Bouin et L. Mail- 

 lard, p. 86. 



* Textbook of the principles of animal histology. U. Dahlgren and W. A. 

 Kepner, p. 7. 



