BLOOD AND VITELLINE VESSELS IN AMPHIBIA 349 
EARLY DEVELOPMENT OF BLOOD AND VASCULAR SYSTEM IN 
AMBLYSTOMA PUNCTATUM 
In specimens of about 2.8 to 3 mm. in length with six primitive 
segments, the mesoderm has extended well about the yolk just 
under the ectoderm. Back some distance from the region which 
will form the liver it is a continuous band, completely encircling 
the yolk cells; forward, in the liver region, it does not meet in the 
middle line and so forms a lateral'band on each side. At this 
stage there is no indication of ventral thickenings and there were 
no blood vessel cells found such as Mollier describes for Triton 
(fig. 1). It is also impossible at this stage to be sure just how 
much of this mesoderm was formed from the ventral lip of the blas- 
topore, although in a general way some of this caudo-ventral 
part may correspond. 
In specimens of about 4 mm. length with 8 to 13 somites there 
is a decided ventral thickening of the mesoderm, both on the 
caudal fused part and the cephalic lateral portions (figs. 2 and 3). 
There is also a more decided indication of the little fold cavity 
just anterior to the liver, with a few cells which seemto have 
come into it from either side. These may be the cells of cardiac 
endothelium such as Mollier describes (fig. 4). There are also a 
few cells on the surface of the yolk, between it and the mesoderm, 
which are in the same position as the blood vessel cells recognized 
by Mollier and Greil and may correspond to them. These stain 
like other mesodermic elements and seem to have been derived 
from the splanchnic layer. They are much smaller than any of the 
yolk cells. 
The ventral thickenings of mesoderm are the first indications 
of blood and there is no difficulty in tracing them through various 
stages into blood corpuscles and vitelline vessels. In this species 
they are at all times well differentiated from yolk and ectodermal 
cells. From the stages I have studied I am not at all inclined 
to regard the early mesoderm as added to by cells from the yolk; 
the earlier cells of the middle germ layer merely multiply to form 
the thickened masses. 
In a specimen of 4.5 mm. or 17 somites, the heart is a mass 
of loose cells between the two pericardial chambers formed by the 
