370 WILLIAM A. HILTON 
A large vessel from the ventral body-wall, double except for a little 
of its cephalic and caudal ends, opens into the capillary network 
of the liver. This is an early stage of the ventral abdominal vein. 
Laterally a series of somatic vessels are easily seen to correspond 
with the earlier parallel systems already traced; perpendicular 
branches from these run into the two lateral parts of the ventral 
abdominal which may be considered to be the last of the series of 
parallels, and in this stage, is not so completely fused into one 
as it is later. The lateral parallel and perpendicular vessels have 
a complicated course similar to that already traced in earlier 
stages. The cephalic portion of this system anterior to the ven- 
tral abdominal vein has broad but rather indirect connections 
with the liver at about the same point as that at which the abdomi- 
nal enters:it. Iam inclined to think that these liver connections 
are part of an earlier one. 
The study of serial sections brings out some points not shown 
in surface views of the living embryos. Probably partly because 
of the larger yolk mass, the mesoderm does not develop in quite 
the same way as in Amblystoma. At the close of segmentation 
the smaller cells at the animal pole migrate about the yolk; some 
of them form a thin, single-layered ectodermal covering, others 
form mesoderm. With the closure of the blastopore many of 
these small cells are carried to the dorsal side of the embryo. 
By this time some of the mesoderm has been formed lateral to the 
notocord, but this does not penetrate far ventrally. A dense 
mass of cells at the caudal end of the embryo, mostly in front of 
the blastopore, but also behind and lateral to it for some little 
distance, represents a region of undifferentiated germ layers. It 
is from this mass that mesoderm is formed in all directions. This 
fuses with the lateral and cephalic mesoderm formed earlier. 
Because of the size of this area and the position of the blastopore 
in it, it is impossible to be sure how much mesoderm is ventral 
and corresponds to that part which Mollier recognizes as giving 
origin to the blood. 
At an early period, when these two chief areas of mesoderm can 
be recognized—that lateral to the notocord, and the caudal mass— 
there is still a large area of yolk not yet covered by a middle germ 
