146 A. A. W. HUBRECHT. 
although there are two intact epithelial surfaces in contact with 
each other, the uterine and the trophoblastic, which do not show 
as yet any wrinkling or any villi. Considering the presence 
of uterine glands, one might expect the surfaces to be 
lubrified by the secretion of these glands, and expulsion of 
the early blastocyst would undoubtedly follow had the swell- 
ing and extension not become limited to the horn only, in 
which, as I have described elsewhere (07, p. 35), it is 
consequently generally very difficult to find the exact situation 
of the embryonic shield. 
The difference in these early arrangements authorises us 
to keep the diffuse placentation of Lemurs apart from that 
of Ungulates. It was not necessarily obtained along the 
same hereditary line of development. 
We have now sufficiently discussed the maximum degree 
of simplification which the placentary phenomena undergo in 
Unegulates, Lemurs, and Kdentates, to which attention had 
also already been called in the preceding chapter. In all of 
them an osmotic exchange between the contents of the 
capillary (not lacunar) circulation in the maternal mucosa 
and the foetal capillaries in the trophoblastic villi is obtained. 
The total surface over which this osmotic interchange takes 
place has become very considerable, and at the same time 
any concrescence between trophoblast and uterine epithelium 
has been quite given up, two intact epithelial layers sepa- 
rating the maternal from the embryonic blood. 
We must now discuss some of the principal deviations from 
the central plan of placentation from which we started in 
opposite directions, viz. in such as bring about, instead of an 
extension of surface for the osmotic exchanges an intensifica- 
tion of the process over a restricted surface. ‘This may, of 
course, be expected in those mammals which have not by an 
increase in the size of the adult (as in many Ungulates), so to 
say, created favourable conditions for surface extension in 
the placentary processes. And, indeed, it is in Rodents, but 
especially in Insectivores and Primates, that we find inten- 
sified conditions as are here alluded to. 
