BLOOD AND VITELLINE VESSELS IN AMPHIBIA 347 
give rise to the vascular system. He traces the peristomial meso- 
derm back to blastula or early gastrula stages. Some of these 
peristomial cells extend forward to join with and form a part of 
the axial mesoderm; the cells which form mesoderm in the ventral 
lip of the blastopore are continuous with the others. The ventral 
part of the mesoderm becomes thickened and forms blood. The 
vascular endothelium of the heart and blood vessels come from iso- 
lated cells which migrate from the thickened masses, or from the 
more ventral axial mesoderm which is partly formed from peri- 
stomial cells as mentioned above. The ventral mesoderm giving 
rise to vascular endothelium and blood is given the name of ‘angio- 
haemoblast.’ Cells separate from the somites in the body region 
and differentiate from a corresponding part of the mesoderm in 
the head region. ‘Those cells which are more or less segmental in 
origin soon lose such an arrangement and, with others, migrate to 
form part of the heart and vascular endothelium. As these vascu- 
lar cells are associated with the sclerotomes they constitute the 
‘angiosclerotome.’ 
Greil and Mollier differ in a number of points: First, they do 
not agree as to the separation of dorsal from ventral mesoderm. 
Mollier, to some degree, takes as a basis for such a separation, 
the splitting of the more dorsal mesoderm into two layers, but 
Greil believes that this has no true significance in this connection. 
According to Greil’s interpretation, it would be very hard to 
distinguish accurately the boundary lines between mesodermal 
cells of the dorsal lip and those from the ventral lip of the blas- 
topore. 
Mollier is in doubt as to how much of the ventral mesoderm 
in early stages is derived from yolk cells or entoderm, but Greil has 
an entirely different idea of these as he considers this mesoderm 
to be from ‘ectoderm,’ or cells on the outside in blastula stages 
and not from the primitive entoderm. 
In a consideration of the question whether the thickened 
masses of mesoderm which go to form blood are derived from local 
thickening by multiplications of cells or derived from migrations 4 
from the region of the blastopore, Greil decides for the former. 
