EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 119 
reveal itself all round (Hrinaceus, man, and Anthropoidea, 
many rodents), or the attachment of the blastocyst to the 
maternal mucosa may be so superficial that the trophoblastic 
proliferations (Fig. 134) serve other purposes than fixation 
(Opossum). Finally, in certain cases no trophoblastic pro- 
liferation is noticed at all (many Lemurs, Sus, Equus). 
At all events, the trophoblastic fixation of the embryo is 
something essentially different from the fixation by means of 
a placenta, although in all cases the definite placenta becomes 
established at a spot where trophoblastic proliferation has 
paved the way, but by no means on all the spots where such 
proliferation has preceded. Some of these regions, as we 
have seen for the hedgehog, and as holds good for other 
mammals, serve the purpose in the form of the so-called 
omphaloidean placentation, others never come in direct 
contact with any vascular area of embryonic origin. 
That part of the trophoblastic proliferation which corre- 
sponds with the spot where the definite placenta is going to 
be developed may be indicated on Duval’s example as the 
ectoplacenta, so that in catarrhine Monkeys and Tupaja there 
is a double ectoplacenta, whereas in the hedgehog we might 
distinguish a ring-shaped omphaloidean and a disc- or 
saucer-shaped allantoidean ectoplacenta. 
Placentation, properly speaking, only becomes a fact when 
this ectoplacenta which is vascularised by embryonic vessels 
and soaked by maternal blood, circulating in vessels or in 
lacunar spaces, has entered into such intimate fusion and 
concrescence with maternal trophospongian proliferation that 
the two tissues can no longer be distinguished, much less be 
separated from each other. In this way there is much to 
say in favour of denying the existence of such a thing as has 
been called ‘“ the diffuse placenta.” Maternal and embryonic 
tissues being in that case so perfectly free and independent 
from each other, that at birth they separate as easily as 
a finger does out of a glove, a placenta cannot possibly be 
said to be present. Fusion of embryonic with maternal 
tissues is its conditio sine qua non, and so we must admit 
