BLOOD AND VITELLINE VESSELS IN AMPHIBIA 343 
Salamandra, he considers the first aortic arches to be formed from 
the endothelium of the heart sac and suggested that perhaps all 
the endothelium of the vessels comes from it. Marshall and Bles 
(90), were of the opinion that the heart endothelium of the frog 
was from entoderm at first, but later became united with the meso- 
dermic cells and lost its original connection. The blood vessels 
are from mesoderm. In most parts of the body they appear as 
irregular spaces or lacunae. They are at first independent of 
each other, but soon extend so as to meet and open into one an- 
other as irregular channels. ‘The cells surrounding these channels 
assume a more definite arrangement and convert them into blood 
vessels. ‘The blood corpuscles are either cells which are inclosed 
within the lacunar spaces from the first, or cells budded off from 
the walls of the vessels into their cavities at a later stage. 
Schwink (91), working with Salamandra, Triton, Rana and 
Bufo, describes the heart endothelium as derived from paired 
masses of cells on the surface of the yolk and states that cells from 
these areas migrate forward. The blood vessel cells he considered 
as arising from a single primary budding mass, lying somewhat 
caudally on the yolk, and from the entoderm. The origin of the 
blood islands he conceived to be on the border line between yolk 
entoderm and mesoderm, if one consider the mesoderm to be 
formed by delamination. He comes to this idea through obser- 
vations on Urodela, where he found as good evidence for the for- 
mation of blood from the entoderm as from the mesoderm, but 
he did not come to a definite decision, although he did not think 
it probable that two germ layers entered into the formation. 
Houssay (’93), working with axolotl, believed that the whole 
vascular endothelium had a segmental origin from the entoderm. 
He called it ‘angiotome’ or ‘parablast.’ He thought that the 
blood originated from the central cells of the first solid anlage of 
the subintestinal vein, while the peripheral cells formed the vascu- 
lar walls; the subintestinal vein being formed out of the long, 
compact mass of a single segmental derivative of the entoderm. 
Nusbaum (’94), studying the Anura, came to conclusions similar 
to those of Schwink with regard to the vascular endothelium. He 
