712 The American Naturalist. [August, 
with the walls of the uterus. Hence, when the uterus is opened 
only the hollow plug and its covering of entoderm can be removed; 
as it makes a two-walled vesicle, it was considered to represent by 
itself the two-layered stage of the blastodermic vesicle. Thus it 
came that Bischoff believed that in various rodents the ecto- 
derm lies inside, the entoderm outside. Bischoff’s observations, 
4, 9, which have been confirmed by Reichert, 77, are correct, but 
the inversion of the layers is apparent, not real. The actual 
homologies were not discovered until the im- 
provements in microscopic technique enabled 
Selenka, 55, 56, and Kupffer, 38, to make 
An 
9 
o 
0,0508 sections of uteri with ova in situ, and in their 
9.8 o 2 : \ : 
| (eS coos sections to find the true outer layer. Their 
OR 996 
^ 
observations removed at once the apparent 
anomaly in the position of the germ-layers. 
Their results have been in the main confirmed 
by Fraser, 20, and extended to another species 
by Biehringer, 5. 
IEn In Mus decumanus the ectodermal cells 
early become a separate spherical mass, thus 
dividing the plug into two parts; a cavity 
appears in each part; these two cavities soon 
aticus, after Selenka. become confluent, and the inner layer of cells 
, having meanwhile developed, the relations 
blastodermie vesicle; become essentially identical with those in 
En, entoderm, ol, Mus sylvaticus, Fig. 21. In Mus musculus the 
outer layer. gaye : = j 
ie 
o 
b) 
so. 
Qo 
29 
"o6 0 
o0 
a Q 
p t is similar, butthere i 
peculiarity that the deckschicht is regularly invaginated at first, 
so as to form a small pit, into which living tissue grows. In 
Arvicola this invagination is more marked and lasts longer, but 
in both cases it is early obliterated. 
Arvicola represents the second modification mentioned above; 
it has not only the invagination to distinguish it, but also the very 
early formation of the cavity of the plug as a fissure between the 
deckschicht and the true ectoderm cells. 
The guinea pig offers the third modification, and is characterized 
by the early complete separation of the plug into its two parts; 
