376 WILLIAM A. HILTON 
other Anura may simply mean that the cells are of similar origin 
in all cases, but there may be difficulty in recognizing them, due 
to slight differences in position and character of the cells in the 
two groups of Amphibia. 
I examined a number of series of Rana sylvatica and found in 
them the first blood apparently being formed from ventral yolk 
cells. This appearance I think was due to the fact that the meso- 
dermal cells and yolk cells in this species are so nearly of the same 
size and stain so much alike in early stages, that it was impossible 
to tell them apart in certain regions. 
The first vascular endothelium in Amblystoma may be formed 
from some of the cells recognized in a position similar to those 
described by Mollier, but for a long time the early blood spaces 
on the yolk are without endothelium and much of the more ven- 
tral vessels(and probably some of the more dorsal ones as well) 
may receive their endothelium from the general ventral thickened 
mass. In Desmognathus, the first blood masses were the first 
indications of blood vessels and evidently, here at least, the cells 
which go to form the endothelium come from these first groups. 
Later developed vessels in this species seem to get their endothe- 
lium from the first areas. I was not able to determine vascular 
cells coming from the somites such as Greil (’08) described. 
The heart was only studied in Amblystoma, where there was a 
close agreement with the work of Muthmann-(’04) and Mollier 
(06), as to the position of the cells which form the endothelium. 
To what extent blood circulates in the early blood spaces which 
have no endothelium is a question, but it is evident in both species 
that there is some circulation in these before they receive their 
lining. 
In Amblystoma at least, the vitelline veins develop first from 
the ventral thickened mesoderm. This is fused in one mass 
behind, but towards the heart it forks to correspond with the 
place where the mesodermal sheet is divided ventrally. Greil 
(708) considers the heart and vitelline veins as continuous from the 
beginning, but in Amblystoma in some early stages there is a 
separation, or at least spaces are developed on the yolk which are 
for a time separate from the heart. 
