348 WILLIAM A. HILTON 
He believes Mollier has no good evidence from cell division fig- 
ures to show that the splanchnopleure contributes to the forma- 
tion of blood vessel cells. ; ; 
As to the question whether or not some cells are added to the 
blood from the yolk, about which Mollier was not willing to 
make positive assertions, Greil believed from his evidence from 
Ceratodus, that the yolk cells have no part in the formation of 
the blood. 
Mollier believed the blood vessel strand, that is to say the 
subintestinal vein, was connected with the heart from the begin- 
ning. Mollier, Rabl and others think of the heart endothelium as 
taking part in the development of the vessels in connection with 
it. Greil believes that the heart is only one part of the general 
vascular endothelium and does not initiate the development of 
the rest. Greil agrees with Mollier as to the paired origin of the 
heart although he does not believe that the cells come from the 
splanchnic mesoderm as does Mollier. 
Mietens (’09) traces the history of the blood in Bufo vulgaris. 
At an early stage the mesoderm is free from the yolk in the for- 
ward part of the embryo; it is not distinct from it farther. back. 
The middle germ layer is continuous with the yolk cells ven- 
trally. Later the ventral mesoderm splits off from the yolk 
It does not increase by a multiplication of cells from the lip of the 
blastopore, but by a separation from yolk cells and multiplica- 
tions in situ. If the ventral lip of the blastopore does contribute 
mesoderm, only a little of the caudal portion is formed in this way 
and the distinction between peristomial and axial mesoderm has 
little significance here. In a later stage there is a secondary ven- 
tral fusion with entoderm. Mlietens believes that Mollier saw 
only such later stages in his work on Bufo and that other early 
investigators in this and later stages considered the blood as aris- 
ing from yolk cells. The blood develops from the ventral meso- 
derm after this fusion with the yolk. Wandering cells arise chiefly 
from the sclerotomes and more dorsal portions of the mesodermal 
sheet; possibly also some of the cells where the blood is formed 
give rise to wandering cells. The parenchyma of the liver forms 
blood and endothelial cells at an early stage. 
