RESPIRATORY NET IN ALLANTOIS OF CHICK 415 



membrane now reminds one strongly of the squamous epithelium 

 of the epidermis. The deepest group of cells are cylindrical or 

 cubic, basophylic and numerous mitoses are found amongst 

 them. Layers of more flattened and less basophylic cells sepa- 

 rate the younger cells from a stratum which shows definite and 

 characteristic changes. Numerous particles of kerato-hyaline 

 appear in the cytoplasm of these cells and transform a whole 

 layer of the epithelial membrane into a stratum granulosum. 

 The outer layers of the membrane undergo in many places a 

 complete cornification. Of course the process has not the regu- 

 larity seen in the epidermis; for example, numerous vacuoles de- 

 velop especially in cells on the surface of the membrane, as seen in 

 figure 3, but the cornification is sufficiently well pronounced to 

 leave no doubt of its significance : the cells of the epithelial-like 

 layer may undergo a cornification; this is a change exclusively 

 characteristic of ectodermal and entodermal derivatives, therefore 

 they must be considered as being of ectodermal and not of meso- 

 dermal origin, (entodermal origin being out of question). 



Interesting changes are observed in the epithelial cubic cells 

 if they are activated at a time when a vascular net has developed 

 above it or in its substance. In the latter case the intensive 

 proliferation of the epithelial cells leads to a discontinuity of 

 individual meshes of the capillary net; the blood corpuscles are 

 then ingested by endothelial cells. Cells of the epithelial mem- 

 brane readily arrange themselves around small accumulations of 

 blood corpuscles or endothelial cells, which soon become necrotic 

 and the whole structure is very similar to an epithelial pearl or 

 to a Hassal's corpuscle. Even more characteristic pictures«arise 

 from the proliferation of the cubic cells when they occupy a 

 sub vascular position. Solid sprouts of ectodermal tissue often de- 

 velop from the deeper layer of the epithelial-like membrane, they 

 push into the mesenchyme of the allantois and here form well de- 

 fined agglomerations of epithelial tissue. Never do the cells of this 

 tissue separate from one another, the tissue may be traversed 

 by numerous ameboid cells, which finally may transform it into 

 a kind of reticulum, like the entodermal tissue in the thymus, but 

 within the allantois they do not break up into individual cells 



