68 ON A PLATYPUS EMBRYO, 



the form of endothelial tubes which may enclose a number of 

 vasifactive cells. 



The vasifactive cells constituting blood islands occur in great 

 numbers opposite the posterior region of the embryo between the 

 more compact superficial layer of mesoderm and the entoderm 

 (figs. 12, 30, vas. c). In the mesial portion of this region the 

 vasifactive cells appear to be differentiating to form vessels, 

 while further out they occur in larger or smaller undifferentiated 

 blood islands. The vasifactive cells possess each a large rounded 

 nucleus with a very thin surrounding laj^er of protoplasm (fig. 30, 

 vas. c). 



Structure of Blastodermic Vesicle. 



The oval vesicle on which the embryo lies is comparable at this 

 stage to a typical mammalian blastodermic vesicle, and forms in 

 some respects a striking connecting link between the conditions 

 oljtaining in the Sauropsida and in the Placental Mammals. 



The for the most part flattened ectoderm cells of the embryonic 

 area pass into the more cubical cells forming the outer layer of 

 the wall of the vesicle. Both ectoderm and entoderm form 

 perfectl}^ continuous layers all round the vesicle (fig. 32). 



The vesicle, as already described, contained a thin albuminous 

 fluid, while below its thin wall there existed a layer of yolk 

 spheres. Sections and preparations of the wall of the vesicle 

 mounted whole show that these yolk spheres are all intracellular. 

 They are contained in large cells — vitelline entoderm cells — which, 

 as has been already stated, are sparsel}'' present among the 

 flattened entoderm cells of the embryonic area, and immediately 

 outside this are more abundant; while throughout the rest of the 

 non-embryonic portion of the vesicle they constitute the entire 

 inner entodermic lining of the latter. 



The vitelline entoderm cells are of great size and are almost 

 entirely occupied by large yolk spheres (figs. 30-33, vit. ent.). 

 Each cell contains a large nucleus rendered somewhat irregular 

 by internal compression by the yolk spheres. The nucleus is 

 generally situated on the side of the cell next the ectoderm (fig. 



