616 J. T. PATTERSON 
to find a common canal for the two embryos developing out of 
the proximal part of the primary bud. 
Each embryonic rudiment is a slipper-shaped structure lying 
at the terminus of a canal (fig. 5), and is in a relatively late 
primitive streak stage. At the anterior end of the embryo the 
medullary plate is well formed, although as yet the medullary 
folds have not become elevated. Posteriorly the embryo ends 
as an irregular, blunt process of mesodermal tissue. 
Fig. 78 shows a transverse section of one of the embryonic 
tubes from specimen No. 276. The section passes through the 
embryo at the level of the extreme anterior tip of the primitive 
streak, and hence cuts the thickened medullary plate, which 
curves upward and inward to become the thin ectodermal layer 
of the amnion. Beneath the medullary plate, and between it and 
the entoderm, are seen the scattered mesodermal cells which 
have been proliferated from the primitive streak. 
Lateral to the embryo is a loose mass of mesodermal cells 
which lie between the entoderm and epithelial-like mesoderm 
of the extraembryonic cavity. In the whole condition of the 
chorionic vesicle the mesoderm was seen to fringe each side of 
the embryonic tube, extending throughout the entire length of 
the embryo proper. It already shows a rudimentary net-work 
of blood-vessels, which represents the beginning of the area vas- 
culosa. I have not worked out the detailed history of the vas- 
cular area, but undoubtedly it arises in each embryo in a man- 
ner similar to that in the typical mammalian ovum from which 
but a single embryo develops. 
The embryonic mesoderm is thickest at the posterior end of 
the embryo, where it gives rise to a series of enlargements which. 
extend for some little distance behind the extreme posterior tip 
of the embryonic tube. This is the portion of the mesoderm 
into which the umbilical vessels and the rudimentary divertic- 
ulum of the allantois later extend; that is, it forms the basis 
for the belly-stalk. 
The further development of each embryo is very similar to 
that of the ordinary mammal, and therefore calls for a brief 
description only. The reader is referred to an earlier paper in 
