94 Walter E. Dandy. 
ever, all the signs of a normal embryo. The question of a primitive 
volk circulation has always been regarded as an established and indis- 
putable fact, based almost entirely upon comparative embryology. 
Recent observations in human embryology, however, have shown 
the human ovum to differ markedly from other comparative forms 
in the very early stages of development. This embryo, together with 
the early embryos which have been described, particularly that of 
Eternod, seems to indicate that the primitive human vascular sys- 
tem differs from the conventional comparative system in that the 
umbilical circulation is the predecessor of the time-honored vitelline 
circulation. 
In studying the vascular system, every capillary was traced as far 
as high magnification would permit. The recent work of H. M. Evans, 
with injections of fresh specimens of young chick and pig embryos, 
has however demonstrated the existence of large hitherto unrecogniz- 
able capillary beds and has shown the comparative inefficiency of 
studying fixed uninjected tissues‘for blood-vessels, even under the 
most favorable conditions. We must therefore always bear in mind 
the possibility of collapsed vessels, and that other things being equal, 
too much emphasis cannot be placed upon negative evidence. Even 
if there should be some small capillary beds, which are unrecogniz- 
able by the microscope in the absence of injections, it would merely 
be evidence in favor of vascular connection between the umbilical 
vesicle and the embryonic circulation, and would not in any way 
affect the question of priority, because of the relatively greater devel- 
opment of the umbilical vessels. A glance at Fig. 15 and Fig. 12 
will conclusively show that the functioning system is umbilical or 
chorionic and that if a connection could be traced through the large 
gap between the umbilical vesicle and the small sprout of the possible 
vitelline vein, it would be comparatively insignificant. Since the 
embryonic vessels are full of blood and there is no apparent connec- 
tion with the blood-forming area in the umbilical vesicle, the ques- 
tion naturally arises, how are we to account for the presence of the 
blood corpuscles? This is explained by finding many beautiful exam- 
ples of endothelial proliferation of blood corpuscles from the capil- 
laries in the chorionic membrane (Fig. 10), probably supplying the 
embryo with blood until the time of connection with the yolk sac. 
