96 Walter E. Dandy. 
vascularity in its early history, irrespective of position, and that from 
this vascular formation, whether from single or multiple points of 
origin, two main primitive vascular areas originate practically syn- 
chronously and develop independently toward the embryo. Eternod’s 
embryo together with the one under consideration, which are the 
two earliest embryos with a complete circulation, would seem to indi- 
cate that the umbilical circulation was the first to attain this fune- 
tion. 
As mentioned before, this embryo agrees in general very closely 
with Eternod’s 1.5 mm. embryo, differing only in a few details. The 
yolk vascular system is developing rapidly and consists of numerous 
large blood islands, forming irregular, disconnected channels and 
masses of blood-forming tissue, situated almost entirely on the ventral 
half of the umbilical vesicle (Fig. 12), extending from the anterior 
to the posterior poles and more marked in the region of the future 
vitelline veins. A few very small blood islands extend along the 
dorsal half of the yolk sac, but never is there any visible vascular 
connection within the embryo and always a considerable distance 
exists between the blood islands and the vessels of the body of the 
embryo. 
The umbilical circulation consists of a simple vascular cycle 
comprising the umbilical veins, heart, dorsal aortee and umbilical 
arteries, with capillary connection in the chorion and chorionie villi. 
A number of villi show the early stages of vascularization, which are 
not limited to the region around the attachment of the bauchstiel, but 
extend anteriorly beyond a point corresponding with the anterior limit 
of the embryo. There appear to be several seemingly independent 
small systems of capillaries in the chorionic membrane, into which 
drain the capillaries of the villi. Although these systems seem to have 
no connection with each other, the difficulty of following capillaries 
in uninjected specimens would make a positive statement impos- 
sible. As mentioned above, many of these chorionic vessels show 
beautiful examples of what appears to be blood formation from 
endothelial proliferations (Fig. 10) and is probably the source of 
the blood for the present circulation. 
Two large independent blood reservoirs (Figs. 8, 9 and 15) in 
the bauchstiel connect distally with the chorionic vessels; proximally 
