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A Human Embryo with Seven Pairs of Somites. 
The chorionic membrane and villi have the characteristic inner 
loose mesenchymatous layer of beautiful branching spindle and. stel- 
late cells with anastomosing processes and a somewhat jelly-lke inter- 
cellular substance; an external double epithelial layer, consisting of 
the inner Langhans layer of small cells with lightly staining nuclei 
and cytoplasm, and the outer syncytial layer which stains more 
deeply with eosin and has larger and more densely staining nuclei, 
but no definite cell boundaries. From the epithelial layer of the 
chorionic membrane and villi, numerous buds develop, some from 
the syncytial layer alone, others from both layers of epithelium. 
These represent proliferation and new formation of villi. The 
mesenchymatous layer of the chorionic membrane contains many 
newly forming capillaries, some of which extend into the villi. The 
details of these will be considered later in the description of the 
vascular system. 
The Bauchstiel. The bauchstiel does not differ from that of 
other young embryos. It consists of loose mesodermal tissue, lined 
externally by a single layer of flattened mesodermal cells. It 1s con- 
tinuous distally with the chorionic mesoderm and proximally with 
the mesoderm of the umbilical vesicle, amnion and body of the 
embryo. It contains the allantois, umbilical arteries and veins and 
their resulting sinuses and branches. 
Umbilical vesicle-—The yolk sae has no stalk but is attached to 
the entire length and breadth of the embryo, with the exception of 
small portions of the anterior and posterior ends, which are covered 
by the short head and tail folds of amnion. At its attachment to 
the embryo, the walls are very thin, consisting of two layers of flat- 
tened cells—mesoderm and entoderm. These walls gradually grow 
thicker distally, due to the degree of development of the blood 
islands, which also cause a great distortion and knotty appearance 
of the mesodermal surface. This vascular development extends 
throughout the whole length of the umbilical vesicle, but is practi- 
cally limited to the ventral or distal half, only a few islands being 
seen in the dorsal half. The greatest development seems to be near 
the center of the vesicle and is apparently developing to connect 
with the future vitelline veins. This is in contradistinction to the 
findings in Eternod’s 1.3 mm. embryo, in which the posterior region 
