352 WILLIAM A. HILTON 
In aspecimen 6 mm. in length and of 22 somites, spaces are evi- 
dent between cells of the connective tissue; whether lined by these 
or others I did not determine. In some specimens, apparently 
younger than this, spaces are found to some degree which do not 
seem lined with cells, but in one individual of this length many of 
the vessels within the body of the embryo have a decided lining. 
In the ventral head region two vessels lead into the heart, the aor- 
tic arches. Dorsal to the alimentary canal in the cephalic region 
two sets of vessels are developed, appearing as spaces in the mesen- 
chyme. There is one each side of the nervous system and one on 
each side of the notochord just above the alimentary canal. The 
more dorsal of these divides a number of times until there is again 
a single pair above and below. The vessels of the more ventral 
pair migrate towards the middle line and nearly disappear, but 
fuse to form the dorsal aorta. Lateral vessels from the others are 
continued down into the somotopleure as spaces between cells 
and, like the heart and aorta, are without blood corpuscles. In 
the caudal region of the heart a large sinus venosus is found; 
continuing into this from both right and left are spaces or vessels 
in the somatopleure. Farther down, the sinus is divided into a 
right and a left portion, the right, the smaller becomes reduced 
and disappears first; these are parts of the right and left vitelline 
veins. Besides the connections with the somatic vessels at the 
sinus, there are indications of vessels out towards the blood masses 
of the yolk ventrally, but in this stage there is no communication. 
Beyond this, there are lateral vessels in the somatopleure, showing 
as wide spaces. Below this region in the splanchnopleure, spaces 
are developed on each side between yolk and mesoderm which, 
although not yet filled with blood, communicate with the lateral 
blood masses. These last appear, as in earlier stages, like thicken- 
ings of the mesoderm, but with a stronger suggestion of blood 
because of the small size of the cells and their prominent nuclei. 
Farther down the two latero-ventral blood masses become fused 
into a large one which becomes smaller as the caudal end is ap- 
proached (fig. 7). Branches from the dorsal aorta are seen here 
and there, some of them seem to communicate with the vascular 
spaces on the surface of the yolk. 
