408 VERA DANCHAKOFF 



the air or the hqiiid by a special membrane, which consists of 

 ecto- or entodermal cells. 



This situation of the respiratory capillary net is merely a 

 special case of the general arrangement of the mesodermal struc- 

 tures in the organisms and necessarily ensues from the primitive 

 development of the inesoderm between the two other germ- 

 layers. To state that the definite localization of this tissue be- 

 comes in itself a differentiating factor would seem to repeat a 

 commonplace. A mesodermal cell acquired definite potencies, 

 and has lost certain others in association with its development 

 not on the surface but between the surfaces. 



Whether the definite localization between two germlayers in 

 the course of development has become for a mesodermal cell a 

 necessary condition for its existence has hitherto seemed sub- 

 ject to little doubt. The fact that both in embryos and in the 

 adult organism, mesodermal structures especially those of mesen- 

 chymal type are separated from the outer environment by ecto- 

 or entodermal membranes is well established through all the ani- 

 mal classes. There still exists a curious exception from the 

 usual relations between the derivatives of the germlayers, which 

 is seen in the arrangement of the respiratory capillary net in the 

 allantois of the chick. Though the definite localization of the 

 mesodermal tissues is their most important character at their 

 appearance, it does not necessarily follow, that it invariably re- 

 mains one of their characteristic features. Mesodermal struc- 

 tures which have appeared between the two primitive germlayers 

 might grow through one of them and develop here further into 

 specific structures. Though instances of such relations between 

 the derivatives of the germlayers have not yet been described, 

 an example is furnished by the capillary net of the chick allantois. 



This membrane has been recently studied by Fiillborn.- I am 

 not, therefore, endeavoring to give in my short note a full ac- 

 count of the development of the allantois itself or of its capil- 

 lary net. The study of a series of allantois between the sixth 

 and twenty-first day of incubation, gave, however, additional 



^ Loc. cit. 



