340 WILLIAM A. HILTON 
complete collection of serial sections loaned me by Professor Gage; 
Desmognathus, because of its lack of pigment in early stages, 
was especially valuable for surface studies of living embryos. 
The difference in the size of the eggs and the differences in their 
early environment made these very interesting for comparison. 
The eggs of Amblystoma are smaller and laid in water. Those of 
Desmognathus are larger and are deposited on land in moist or 
nearly dry situations. A few series of frog embryos were studied 
but no particular attention was given to any part of development 
except the early formation of the blood. 
The blood and blood vessels were considered to be derived 
from mesoderm, but a detailed cytological study of these cells 
was not followed out. This would be necessary in a consideration 
of the question of an angioblastic layer. The development of 
the heart and later blood vessels are not included in this study. 
There are many opinions as to the origin of the vascular system 
in Amphibia. Probably no greater differences of interpretation 
are encountered in the literature than those dealing with the germ 
layers involved in various parts of the system. Among investi- 
gators favoring the entodermal origin of the heart endothelium 
were: Reichert (40), Remak (750), Goette (75), Rabl (87), 
Marshall and Bles (’90), Sehwink (91), Houssay (’93), Nusbaum 
(94), Brachet (98). Most of these also considered the vascular 
endothelium in general as entodermal. Some of those who 
believed in a mesodermal origin for the heart endothelium were: 
Van Bambeke (’67), Salensky (95), Brachet (’03), Johnston 
(03), Muthmann (04), Moller (’06), Greil (08). The vascular 
endothelium was considered to be of mesodermal origin by Mar- 
shall and Bles (90), Brachet ’03), Mollier (06), Greil (’08). 
In respect to the blood there were also the two views as to its 
origin. Some of those favoring the entodermal origin were: 
Goette (75), Schwink (’91), Houssay (’93), Brachet (98). Those 
who gave it a mesodermal origin were: Marshall and Bles (’90), 
Brachet (’03) in Anura, Mollier (06), Greil (?08), Mietens (’09). 
Many older workers were of the opinion that the heart endo- 
thelium had an unpaired origin, but Schwink (91), Houssay 
(93), Mollier (06) and Greil (’08), believe in a paired anlage. 
