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 layer 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 
obtaining 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 
perfectly 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 sparsely 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. 
