356 WILLIAM A. HILTON 
Farther down, right and left blood masses are still definite; they 
are rather large and are not separated as far as in the specimen 
previously described. They fuse into a median mass at a more 
cephalic level; farther forward this becomes smaller and disap- 
pears. Numbers of well marked vitelline arteries communicate 
with the yolk lacunae, especially in the caudal region. 
In another specimen of the same length the caudal part of the 
sinus venosus leads into a right and a left portion; the left or vitel- 
line vein early receives branches from the dorsal body-wall veins or 
spaces and has some communication with yolk spaces and then 
disappears. The right communicates with veins of the body-wall 
with little or no connection with the yolk vessels. 
In another specimen, apparently younger, the left vitelline 
disappears first, the right grows larger farther down and seems to 
have some communication both with vitelline and somatic vessels, 
the latter being best marked. The two lateral ventral blood 
masses are not greatly differentiated, but may be followed down- 
ward a considerable distance as two distinct groups until they 
approach each other and fuse in one at the middle line; towards the 
caudal end this becomes small and disappears. 
In a specimen 7 mm. long, the right and left branches from the 
sinus are marked; the right is smaller and communicates in its 
upper region with somatic vessels, and farther down seems to run 
into a small right yolk channel. The left in a similar way com- 
municates with both of these sets of vessels. 
Blood may be found in the sinus venosus in all of these later 
stages in considerable quantities; it probably comes to it from its 
vitelline connections, and in these early stages cannot be forced 
out of the heart into the small and as yet only partially developed 
arteries. 
In another specimen 7 mm. long the right branch from the sinus 
venosus is much smaller than the left, which is soon brought into 
communication with the large viteline spaces on the left side as 
well as the somatic vessels. ‘The right in this specimen gives little 
indication of connections with yolk vessels, but is broadly con- 
nected with somatic. Farther towards the tail however, after 
