A Human Embryo with Seven Pairs of Somites. 89 
anterior vesicle is long, wide and deep with edges everted and pro- 
jecting outward over the anterior and lateral walls of the anterior 
body elevation. The walls of the second and third vesicles have a 
tendency toward inversion and the enclosed vesicles are more sharply 
defined laterally than the first. The brain passes insensibly into the 
spinal cord, which is much smaller in all diameters, but 1s nowhere 
closed. Posteriorly the medullary groove forms a shallow dilatation 
eradually fading into the flattened surface of the primitive streak. 
(Plate IV.) 
No neurenteric canal is visible, though present in the younger 
embryos of Frassi, Graf Spee and Eternod and _ possibly the 
Kroemer-Pfannenstiel Klb. No traces of spinal or cerebral ganglia 
or nerves are visible. No suggestion of anlage of the lens, optic or 
otic vesicles could be detected. 
Primitive streak. Posteriorly and dorsally in the region of the 
termination of the notochord and neural groove, ectoderm and meso- 
derm gradually losing their individual morphological characteris- 
ties, fuse to form a mass of a single variety of simple undifferenti- 
ated cells, which extends to the posterior termination of the embryo. 
This is the primitive streak (Fig. 7). The entoderm also seems to 
be a part of this mass since its cells are directly continuous with it, 
although the characteristic lining of the hindgut is still maintained 
throughout. There is no groove except a very shallow flattened 
dorsal depression which is gradually lost posteriorly and which 1s 
continuous anteriorly with the neural groove. This primitive streak 
region suggests a storehouse of simple undifferentiated cells supplying 
mesoderm, ectoderm and entoderm in the earlier stages, later becom- 
ing differentiated into the characteristic morphology and arrange- 
ment of the different layers. 
ENTODERM. 
Entoderm lines the ventral surface of embryo, fore and hind 
gut, allantois and the inner surface of the umbilical vesicle. In 
the ventral entoderm of the embryo is a median longitudinal 
groove corresponding to the location of the notochord and caused 
by its adherence to the ectoderm of the medullary groove. On 
either side of this groove and parallel to it is a ridge caused by 
