BLOOD AND VITELLINE VESSELS IN AMPHIBIA 341 
Rabl (’87) described the development of the endothelium of 
some of the vessels as outgrowths from cardiac endothelium and a 
number of other observers, as Brachet (’98), and Mollier (’06) to 
some degree, favored this view of the origin of some of the vessels. 
Greil (’08) believes the vascular endothelium has a similar origin 
to that of the heart and that it arises at about the same time. 
The origin of blood vessels independently of the blood, seems 
to have been shown in a number of cases. Schwink (’91), Houssay 
(93), and Brachet (’98) were among the first to distinguish be- 
tween blood-forming cells and the cells which develop into vascu- 
lar endothelium ; 
All the earlier writers, up to the time of Mollier (’06), recognized 
the general ventral region of the embryo which gave origin to 
the blood. Some considered it to be mesodermal, others ento- 
dermal. Molier traced this back to early stages and interpreted 
the material as arising from the mesoderm of the ventral lip of the 
blastopore; the vascular and cardiac endothelium arising in part 
from the splanchnic mesoderm. Greil (’08) traced the mesoderm 
‘still farther back in Amphibia and other vertebrates and recog- 
nized special areas of cells on the surfaces of blastula stages which 
afterwards gave rise to the blood and a large part of the vascular 
system. He also recognized some of the vascular cells as having 
a segmental origin, from somites. Houssay, at an earlier time 
(93), believed in a segmental origin of the vascular system, but 
considered it to be from entoderm. 
The way in which the capillaries and early blood vessels develop 
cannot be easily learned from the literature. Some of the early 
investigators believed than many of the first vessels were simply 
spaces in connective tissue, the endothelial walls being formed 
later from neighboring or surrounding mesenchymal cells. Others 
described the spaces, but believed that more or less isolated cells 
migrated in and gradually formed endothelium. Such was the 
idea of Goette (’75), and of Mollier (06) in more recent times. 
Whether or not there was a circulation of blood before all the 
endothelium was formed, was not determined in many cases, but, 
judging from the vitelline circulation in some teleosts and from 
the descriptions and figures of Mollier, there seems to be some 
