BLOOD AND V{TELLINE VESSELS IN AMPHIBIA 371 
layer, especially on the ventral side towards the head end. After 
the mesoderm has penetrated over a large area, between the ecto- 
dermic covering and the yolk, it becomes very thin except near the 
body of the embryo; so thin that it is difficult to make out more 
than a very slender line of cells. In the later development of 
mesoderm there seems to be little evidence that the yolk or ento- 
derm contributes to its formation, for the yolk cells are large and 
have large granules, but it is possible that some small cells left 
behind by the migration from the animal pole may earlier or 
later join with the mesodermal ones. 
In sections of embryos of about 13 somites (such as figure 34) 
the first indication of blood is shown by little lateral thickened 
masses of nuclei; these, I believe, are derived from mesoderm 
which has penetrated under the ectoderm at an earlier stage, but 
is now for the most part so thin as to be hardly noticeable (fig. 38). 
The first mesodermic indications of blood correspond in position 
to the first signs of blood island seen in surface views. These are 
compact groups of nuclei in which I was at first unable to see 
anything of a vessel wall (fig. 39). There is usually at least one 
longer thickened mass on each side but it was not considered to be 
a particular part of the early mesodermal sheet. Later, smaller 
masses become more numerous and may be somewhat isolated at 
first, but soon join one another, as the surface views indicate. In 
later stages other masses develop over the yolk, while some of the 
earlier ones become larger and may begin to show signs of a cavity 
as certain of their cells develop walls of the vessels (fig. 35). 
By this time the body of the embryo has developed blood vessels 
but they are without blood. Spaces occur on the yolk and for a 
time they are without endothelial linings (figs. 40, 41, 42); the 
walls afterwards gradually form in them from special vascular 
cells which probably bud out from the blood masses. The for- 
mation of spaces on the surface of the yolk goes along with the 
development of additional blood masses not inclosed in an endo- 
thelium, but when the blood is circulating, such spaces are probably 
penetrated by corpuscles almost as soon as formed, although it 
may be some time before the spaces are lined by endothelium. 
Figure 40 shows a space without endothelium but with blood 
cells. 
JOURNAL OF MORPHOLOGY, VOL. 24, No. 3 
