STUDIES IN MAMMALIAN EMBRYOLOGY. 337 
towards the final enucleation of the after-birth out of the re- 
maining decidual structures. Fig. 49 gives an idea of the 
aspect of the top of the crescent in these later stages, and of 
the character of the newly formed epithelial cells. It moreover 
shows that this proliferated epithelium is not strictly limited 
to the surface opposite the decidua reflexa, but that the portions 
of the decidual tissue, which are as yet not yet stretched into 
the membranous shape, and which may be called the margin 
of insertion of the decidua reflexa, are temporarily also covered 
by distinct epithelial cells, which later on share in the retro- 
gressive process when the transition from stage 35—36 takes 
place. However, we may feel sure that even at the moment 
the after-birth is severed from its attachment by a rupture that 
suddenly and violently unites the two tips of the crescent, a 
distinct epithelium covers for a certain distance that part of 
the convex surface of the after-birth corresponding to the 
point up to which the proliferation has reached. The convex 
surface of the decidua reflexa, the epithelium of which de- 
generates (fig. 37), can still be seen to be covered by that 
epithelium up to a stage between figs. 33 and 34. Later on it 
has disappeared, and reflexa, trophospongia, and diplotropho- 
blast are then reduced to what is indicated in fig. 45. 
III. The Physiology of the Hedgehog’s Placentation. 
In the preceding chapter I have purposely restricted myself 
to a description of the anatomical details and the successive 
changes that are noticed in the tissues of the pregnant uterus 
and in the developing blastocyst. Even those structures 
there described that evidently derive pre-eminent significance 
from nutritory purposes, were only superficially touched upon, 
in order not to confuse observation and description with in- 
ference and speculation. And so now we will have to consider 
more carefully under what conditions of nutrition and respira- 
tion the embryo of Erinaceus may be said to be placed during 
the different stages of pregnancy. There can be no doubt that 
