410 VERA DANCHAKOFF 



derm, and so again reduced to its primitive two layers of ento- 

 derm and mesoderm. 



The two figures 1 and 2 illustrate the process of the develop- 

 ment of the capillary net on the surface of the allantois. At no 

 stage does the epithelial ectodermal membrane degenerate; it is 

 invariably present, but it changes the superficial position which 

 it occupied in earlier stages and is found in later stages below the 

 capillary net. There is little doubt, that the epithelial layer of 

 cubic cells observed by Fiillborn and thought to be of mesoder- 

 mal origin is the ectodermal layer of the serosa missing in his 

 description.- 



Figure 1 represents the superficial layers of the outer sheet 

 of the allantois. The ectodermal membrane is at this stage 

 well defined; its constituent cells are easily identified; their 

 large light nuclei with a well developed nucleolus and a net of 

 chromatic filaments markedly differ from those of the mesodermic 

 cells; their cytoplasm, stained with Eosin Azur, takes a purplish 

 tint, while the mesodermic elements show a blue-greenish color. 

 No definite boundaries are seen between the cells of the ecto- 

 dermal layer, and their respective limits are merely indicated by 

 regions of somewhat acidophylic cytoplasm. The more superfi- 

 cially situated ectodermal cells appear flattened. Numerous 

 mitoses are encountered. 



Figure 1 represents only the superficial parts of the outer 

 layer of the allantois. A branch of a deeper situated vessel runs 

 perpendicularly to the ectoderm membrane, penetrates it and 

 here resolves itself into numerous meshes of a capillary net. 

 The meshes are situated in the substance of the ectodermal 

 membrane and are intimately surrounded by ectodermal cells. 

 They have their own walls which consist of a thin endothelium 

 with numerous nuclei. These nuclei are small and flattened, 

 possess numerous small chromatic particles but no* visible nu- 

 cleoli and take a marked greenish tint. The meshes of the net 

 situated in the ectodermal layer are derived from the vessels 

 which grow into and pierce, the ectoderm. They are filled by 

 circulating blood in greatest part by erythrocytes, though granu- 

 lar leucocytes and occasionally hemoblasts are seen. 



