A Human Embryo with Seven Pairs of Somites. of 
the smaller gives off the paired umbilical veins; the larger is formed 
by the confluence of the umbilical arteries. 
The umbilical veins run in the dorsolateral portion of the body 
of the embryo, at the point of origin of the amnion. The left vein 
gives one, the right vein two, very short branches to the amnion at 
the level of the posterior region of the embryo. Nothing comparable 
to the sinus ensiforme of Eternod’s 1.3 mm, embryo was observed. 
While still in the dorsal position and just before its course ventral- 
ward, each umbilical vein gives off a short branch which runs ven- 
trally through the body of the embryo toward the umbilical vesicle. 
On one side it terminates before reaching the yolk sac, on the other 
side it runs a short distance in the wall of the yolk sac and seems 
to end blindly. On each side a couple of spaces, presumably capil- 
laries, although they do not contain any blood, are in the immediate 
vicinity of these short branches, but no connection can be detected. 
These short branches and possible capillaries may be the beginnings 
of the vitelline veins, although the dorsal point of origin from the 
umbilical veins is rather against this view, unless there is a subse- 
quent ventral wandering, as has been observed in many other vessels. 
This is the only visible evidence of the possibility of the vitelline 
veins. (Plate IV.) 
The umbilical veins now run ventrally around the celom and 
unite to form the heart just anterior to the origin of the foregut. 
It is a simple tube of endothelium throughout. From its anterior 
extremity one complete aortic arch is given off on each side (Fig. 
- 1) and in addition, four short stubs on the right and two on the 
left sides, which represent rudiments of future aortic arches. No 
corresponding buds are given off by the cephalic portion of the 
dorsal aortee to connect with these. The heart is a step in advance 
of that of the Kroemer-Pfannenstiel embryo, which has one aortic 
arch on each side, but a paired heart is still present. Eternod’s 
1.3 mm. has a slightly higher development, in that three and _ pos- 
sibly four complete arches are present on each side. 
The dorsal aorte, paired throughout, begin anteriorly in a small 
dilatation and terminate caudally at the posterior border of the fore- 
gut in a much larger dilatation, from which the umbilical arteries are 
the direct continuation (Plate VI). Each aorta gives off a series of 
