350 WILLIAM A. HILTON 
separation of the two mesodermic layers in this region (fig. 5). 
This group seems to have been added to from the visceral layer, 
especially in several specimens in stages between this and the last 
one described. At this time, as in one earlier specimen (figs. 2 
and 5), a few rather small isolated cells on the surface of the yolk 
in the dorsal and lateral regions begin to look more like early 
blood vessel cells, while the ventral thickening of mesoderm has 
become much more marked: and, in the regions where blood will 
be formed, the cells are much smaller than adjoining yolk cells, 
and they begin to look more like blood corpuscles. The mass is 
median behind, and forked and lateral in front. It really is the 
thickened ventral mesoderm, and in later stages passes directly 
into the vitelline vessels. 
In a specimen 5.5 mm. in length, with 19 somites, the heart 
is a solid mass of cells, and the pericardial cavity well formed. 
There are as yet no blood vessels and the ventral portion of the 
mesoderm is thick, while laterally it has become reduced to a 
thin line of cells (fig. 6). 
Fig. 1 Section of an embryo Amblystoma, 2.8 mm. long, with six somites, show- 
ing m, mesoderm as yet not thickened. X 25. 
Fig. 2 Section of embryo Amblystoma 3.5 mm. with 8 somites. The section is 
through the liver and far enough forward to show the mesoderm separated on each 
side. The mesoderm is thickened ventrally and some early vascular cells, », 
on the left between mesoderm and yolk. X 25. 
Fig. 3 Section through the same embryo as figure 2, only farther towards the 
tail end. It shows the beginning of the thickening of ventral mesoderm, n. X 25. 
Fig. 4 Section through an Amblystoma, 3.5 mm. with eight somites, showing h, 
the early heart-forming cells. X 25. 
Fig. 5 Section through the heart region of an Amblystoma 4.5 mm., showing 
also some vascular cells (v) on the right. X 25. 
Fig. 6 Section of Amblystoma, 5.5 mm. long, 19 somites, mesoderm thickened 
below. X 25. 
Fig. 7 Section through an embryo of 6 mm. or 22 somites, blood formed of ven- 
tral mesoderm. X 25. 
Fig. 8 Section through an embryo 7 mm. long, showing two blood vessels, 
b. X 25. 
Fig. 9 Section through an embryo about like figure 8, with blood spaces and 
blood corpuscles on the surface of the yolk. x 25. 
Fig. 10 Sections through an embryo which shows lateral cutaneous vessels, J. 
The yolk vessels at this stage are lined with endothelium. X 25. 
Fig. 11 Section of late larva showing lateral cutaneous vessels, and the ventral 
abdominal vein, a. X 25. 
