344 WILLIAM A. HILTON 
also believed that cells migrated forward from these first masses. 
He believed that the blood and vascular endothelium were en- 
tirely from yolk entoderm. 
Salensky (’95), regards the endothelium of the heart of frog as 
derived from the middle germ layer, and arising from mesoderm 
on either side of the heart area. 
Morgan (’97), does not particularly favor either the mesodermic 
or entodermic origin of the heart endothelium. He makes the 
statement however, that if the cells forming this endothelium 
arise from the ventral wall of the archenteron, as has been de- 
scribed by some, they have a different origin from other parts of 
the heart. He describe the first blood vessels as lacunae in con- 
nective tissue which are lined by mesodermic cells. The vessels 
arise in part as outgrowths of already existing vessels and in part 
as isolated lacunae in the mesoderm. ‘The walls in either case are 
from the same germ layer. 
Brachet (’98), found in Triton an unpaired median mass of 
tissue between the mouth and the liver, arising from the gut-wall 
and forming the walls of the primitive heart cavity. He consid- 
ered the cranial yolk vein to be partly developed from sprouts from 
the heart anlage and considered the general view of Rabl quite prob- 
able. He believed in an entirely entodermic origin for the blood 
in Urodela and clearly distinguished between blood cells and blood 
vessel cells. In 1903 Brachet extended his work to Anura but 
considered the heart endothelium of frogs to be derived from meso- 
derm and not entoderm as he had found in Triton. He recog- 
nized very clearly in the frog the same area of blood-formation 
as the earlier writers, Goette and Schwink, and considered it to 
be from mesoderm. Although he reviews his work on Triton, he 
finds nothing to change in his earlier paper, but a short study of 
axolotl convinced him that it is not possible entirely to exclude 
the ventral mesoderm from the formation of the blood-vascular 
apparatus. 
Johnston (’03), found the heart endothelium of an unknown 
species of amphibian to be strictly mesodermic, although not at 
any time identified with the undifferentiated mesoderm. That 
is, it was split off from the entoderm in the same manner as the 
