602 J. T. PATTERSON 
of the buds are undergoing a rapid increase, as is evident from 
the numerous mitotic figures seen here. 
On the lower side of the ectodermal vesicle the wall is composed 
of a single, thin layer of cells, and below. this is the layer of 
mesoderm. It will be recognized that these two layers together 
constitute the true amnion, out of which the amnion of each 
embryo will eventually arise. 
The mesoderm of the amnion is a portion of the mesothelium, 
the origin of which was described in a preceding section. The 
extraembryonic body cavity occupies the entife space lying be- 
tween the amnion and the mucosa, and only traces (fig. 22) of 
the partition of the two original mesodermal vesicles remain to 
tell the history of this large cavity. 
The outer (upper) surface of the chorionic vesicle consists of 
entoderm. This unique condition has been brought about 
through the disappearance of the chorionic ectoderm (hinfalliges 
Ectoderm of Fernandez ’09), which has sloughed off. Its dis- 
appearance may occur prior to this period (fig. 20), or it may 
persist even until the four embryonic rudiments are established 
(fig. 23). The ectoderm breaks off just beyond the point where 
the entoderm unites to its inner surface (fig. 22, x), and thus 
exposes the entire outer surface of the entoderm to the cavity 
of the uterus. This condition persists throughout the entire 
period of gestation, in a manner to be described later. 
Prior to the formation of the primary buds, the ectoderm and 
the adjacent entoderm remain distinct from each other, as 
seen in such specimens as those shown in figures 58 and 59; but, 
upon the development of the buds, these two layers are brought 
into intimate contact, which at certain points amounts practi- 
cally to a fusion between the two layers. These points are situ- 
ated just above the central area of each primary bud, and 
therefore mark the general region of the primordia of the future 
embryos. : 
No important changes in the extraembryonic mesoderm appear 
to be taking place at this time. It retains its epithelial-like 
character and no cell proliferations are found. A few mesoder- 
