No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 405 
believe it to be identical with Rauber’s layer, and shall speak 
of it as such. According to Duval this Rauber’s layer disap- 
pears over the embryonic disc in the Chiroptera much as in 
the development of the rabbit and the field mouse. This does 
not necessarily contradict Selenka’s observations on Pteropus, 
for the house mouse begins to develop like the field mouse, 
but continues during the early stages in the same manner as 
Pteropus does. 
In the next stage the ectoderm has been converted into a 
hollow mass of cells, Fig. 2, rather by a process of absorption 
than by an invagination, as I have expressed it in the diagram. 
The entoderm lines the whole interior of the egg, and surrounds 
the ectoderm of the amniotic cavity. The ectoderm of the 
exterior of the egg, Rauber’s layer, is again thickened over the 
embryonic mass to form the placenta, as Selenka calls it, or 
the 7réger, if we were discussing rodent embryology. 
In the next stage, as expressed in Fig. 3, the mesoderm is 
beginning to form, and has extended completely over the 
amnion and partly over the umbilical vesicle. The entoderm 
has retracted itself and touches the ectoderm ; only the chorda 
dorsalis is yet to form. Between the amnion and the placenta, 
or the Zager portion of Rauber’s layer, there is a marked 
space, and the mesoderm does not come in contact with it. The 
allantois grows as a bag into this space and attaches itself to 
the thickened part of the ectoderm, as shown by Géhre! in his 
figures. In the figure 3 accompanying Gohre’s paper he shows 
the vesicular allantois attached to the support of the chorion 
(black portion of my Fig. 3) leaving on either side of the embryo 
acoelom. The allantois carries the mesoderm and vessels to 
the villi of the chorion, and these in turn are imbedded in the 
decidua of the uterus. In so doing the ectoderm of the chorion 
receives a second layer of epithelium, and I believe that this 
must account for the two layers of epithelium we have on the 
chorionic villi of the human ovum. There has been much 
written on the subject of the double layer of epithelial cells 
of the human chorion, and I think that a glance at Godhre’s 
figures 3 and 4, on Pteropus, as well as at Selenka’s figures 11 
1 Gohre: Selenka’s Studien, etc., 1892, p. 218. 
