BLOOD AND VITELLINE VESSELS IN AMPHIBIA 345 
rest of the mesoderm, only somewhat later and.separately. This 
view of the origin of the endothelium might well be applied to 
other cases in which an entodermic origin of the heart has been 
given. 
Muthmann (’04), does not agree with Brachet’s account of the 
origin of the urodele heart from the entoderm, and he is of the 
opinion that Brachet confused the early stage of the thyreoid 
which is from entoderm, with the heart which is from mesodermal 
cells. These cells separate on the middle line and fill in a little 
cavity under the entoderm and between it and the ectoderm, just 
in front of the liver. 
Mollier (06), in his article in Hertwig’s Handbuch, shows in 
Triton a little more clearly the origin of the heart from splanchnic 
mesodermal cells in either side of the body. These cells pene- 
trate into the fold cavity described by Muthmann and form the 
endothelium. In Bufo he agrees in the main with Brachet, but 
finds no median anlage. He believes the cardiac endothelium 
arises from paired groups of cells derived from the visceral peri- 
cardial plate. 
In Triton embryos of twelve somites Mollier recognizes early 
blood-vessel cells which separate more or less dorsally from the 
mesoderm on the surface of the yolk. Later there are developed 
in connection with these, or independently of them, lacunae on the 
yolk just under the splanchnic mesoderm, and the vascular cells 
penetrate into these and form the endothelial lining. In Bufo a 
similar development is shown, although the vascular cells are not 
so unquestionably of mesodermal origin. According to Mollier, 
Maurer (’92) saw in a Siredon embryo with fifteen somites two 
groups of cells, one of which he called connective tissue, the 
other, a ventral group, which he described as coming from ento- 
derm and giving rise to the subintestinal vein. Mollier, however, 
believes the upper of these were the blood vessel cells similar 
to those which he describes, and the lower, heart forming cells. 
Houssay (’93) saw this ventral group and regarded it as the 
anlage of the subintestinal vein. “ 
In Triton, Mollier traces the history of the mesoderm which 
arises from the ventral lip of the blastopore up to a time when it 
