BLOOD AND VITELLINE VESSELS IN AMPHIBIA 305 
The large sinus venosus, just back of its connection with the 
’ veins on either side and back of its partial connection with the 
yolk spaces, is divided into two parts, a right and a left; the right 
is smaller and looses its cavity sooner than the left. Probably 
a great part of the caudal portion of the sinus may represent right 
and left vitelline veins. 
In a specimen of about 22 somites and 6.5 mm. length, the heart 
leads into a large sinus venosus but not so large as earlier; it has 
a few blood corpuscles in it and in cross section is just over the 
backward extension of the liver. Farther down, the sinus becomes 
divided into two portions—a narrow left and a larger right vitel- 
line vessel; the left soon has communication with the vascular 
spaces over the surface of the yolk as well as those up in the body- 
wall. On the right side the vessel remains large for a time, with 
little or no communication with spaces on the yolk, although it 
has broad and extensive connections with vessels which appear 
as large spaces in the body-wall. Farther down, beyond the 
disappearance of the portion just followed in the right, a yolk 
space comes into view, and on both sides the yolk vessels or spaces 
may be followed. In some places there are depressions on the 
surface of the yolk under the thin mesodermal sheet and these 
appear without lining and with few or no blood corpuscles, yet 
the yolk cells seem crowded in as though some fluid had pressed 
against them. Others of these spaces come to be lined by cells 
with large nuclei but very flat cell bodies, these cells are perhaps 
derived from isolated ones described earlier. 
Farther down, the two blood masses occur in depressions on 
the surface of the yolk, communicating broadly with submeso- 
dermal spaces on it. The two ventral blood masses have become 
more like vessels and continue back on the yolk, separated farther 
from each other than in earlier stages. Only a small mid-ventral 
vessel some distance back remains of the median fused mass. 
In another similar stage, the left vitelline part of the sinus 
venosus is short, but it has communications with somatic and 
yolk vessels. The right remains longer and evidently communi- 
cates with body-wall vessels and then disappears. At about this 
level a mass of blood occurs ventrally on the right and left sides. 
JOURNAL OF MORPHOLOGY, VOL. 24, No. 3 
