360 WILLIAM A. HILTON 
sels in the mid-ventral line and ends in the liver; that is, it retains 
some of the original connections of the somatic vessels which arise 
in early stages in connection with the sinus venosus. 
The development of the blood and early blood vessels of Ambly- 
stoma may be summarized as follows: 
1. Cells, which apparently arose from mesoderm from both 
sides of the body, are found in the fold cavity recognized by Muth- 
mann just in front of the liver. These were the earliest indica- 
tions of cardiac endothelium and were recognized in a large and 
complete series of early stages cut in all planes. These seem to 
correspond to similar cells recognized by Mollier. 
2. Mesodermal cells next to the yolk were recognized in a 
similar position to those described by Mollier, and may corre- 
spond in part to some of the blood vessel cells. 
3. There comes to be a median ventral thickening of the meso- 
derm in the caudal region of embryos of about eight somites. Jn 
the cephalic region, where mesoderm does not unite across the 
middle line, this mass is continued on each side as a lateral thick-" 
ening. The larger portion of the mass becomes transformed into 
blood corpuscles, but some of the cells develop into endothelium 
of blood vessels. 
4. This area becomes more prominent in later stages and the 
cells are formed into blood corpuscles and they sink into the yolk- 
masses, but at all times they are easily distinguished from the 
larger yolk cells. As the embryo grows in length the fused caudal 
end of this area becomes farther removed from the heart and, as 
other vessels are developed, it comes to be much less evident. 
5. All along the sides of the body under the mesodermal cover- 
ing and on the surface of the yolk, spaces begin to be formed, 
distinct from the blood masses at first, later in communication with 
them. Somewhat later, mesodermal cells form endothelial lin- 
ings for these spaces. In early stages the cephalic portions of the 
vitelline vessels seem to have somatic connections. 
6. At about the time of the development of blood spaces on 
the surface of the yolk, vessels are developed within the body of 
the embryo. The heart has been formed into an endothelial 
tube and is in communication with the vessels of the body. Cau- 
