STUDIES IN MAMMALIAN EMBRYOLOGY. 361 
intensely, forming numerous karyokinetic figures. Club- 
shaped and conical projections of epithelial origin not only 
project into the lumen of these vessels, but the vessels them- 
selves increase considerably by solid proliferation of the epi- 
(endo-) thelium in a centrifugal direction. This increase takes 
’ place to a small extent in a radial, but to a much larger extent 
in a transverse direction ; so that gradually there arises at this 
outer border of the decidual layer [trophoblast of the present 
paper] a special, though not rigidly defined, layer of elongated 
epithelial sacs or strings. These strings are partly without any 
lumen, partly they are filled with blood, between which and the 
epithelial surroundings there is no special wall whatever.” 
Frommel furthermore gives ample details about the com- 
munication of these lacunar blood-spaces with the uterine 
vessels on the one hand, and with the trophoblast-lacune on the 
other, that fully correspond with what obtains in the hedgehog. 
Aiso it is interesting and affords further support to the identifi- 
cation here advocated, that he describes special modifications 
which certain elements of the bat’s trophospongia (his “ epi- 
thelial layer”) undergo, and which are also noticed in the 
hedgehog’s trophospongia, more especially in that outer layer 
of it where the so-called “ deciduofracts ”” are most numerous. 
I will not here go in for a more detailed comparison, but may 
not omit to mention that in this layer Frommel is strongly 
inclined to admit a free formation of red blood-corpuscles out 
of part of this cell-material. I find no indication that he has 
given any attention to the possibility of phagocytic processes 
taking place in this region. At all events the layer which he 
marks by Bi. 6. in his fig. 21 is undoubtedly the outer layer of 
the bat’s trophospongia, and also as such comparable to the 
layer of the deciduofracts in the hedgehog’s trophosphere. 
Outside of this layer, both in the bat and in the hedgehog, 
there is that part of the maternal mucosa which has arisen as 
a special proliferation at the time of conception, and which in 
both these mammals undergoes a very similar metamorphosis. 
The cells, polygonal at the beginning, with round or oval 
nuclei, become more and more stretched as the blastocyst and 
