32, A. A. W. HUBRECHT. 
is by some looked upon as closely related to monkeys and 
man, but of which the placentation is so utterly different. 
And so we will have to look out for a probable cenogenetic 
explanation of these cases of so-called diffuse placentation, 
which were already discussed above (p. 113). 
The first condition that should be fulfilled by a natural 
scheme of placental phylogeny is this, that the different 
families and orders of mammals should fit into it according 
to the degrees of relationship that have already been estab- 
lished by means of the other systems of their organisation. 
And in making an attempt in this direction it is natural that 
we should first ask what is the nature of the placentation in 
those mammals that may be looked upon as representing the 
more primitive types—the Didelphia, the Insectivores, the 
Rodentia, the Primates? We then find, as we have already 
in part discussed above, that the Didelphia furnish very 
conclusive evidence of their being very specialised descendants 
of the placental mammals, that even in those, in which there 
is no more any real placentation as in the Opossum, there 1s 
yet a very active proliferation of the trophoblast, and that in 
those which do retain placentation, or the traces of it, this 
placentation can be omphaloidean (Dasyurus) or allantoidean 
(Perameles). Finally that in this latter case intimate fusion 
on a phagocytic basis comes about between embryonic and 
maternal tissue.! 
If we examine the two other orders of more primitive 
Mammalia, that have been submitted to a more extensive 
inquiry as to their placentation, the Insectivores and the 
Rodents, we are immediately struck by a fact of prominent 
importance as compared to what we find in the so-called 
higher orders, the Carnivores, Ungulates, Chiroptera, etc., viz. 
a most considerable amount of diversity, both in the general 
outlines and in the details of placentation. ‘his is well 
| This holds good, whatever view we may be inclined to share: J. P. Hill’s 
that the trophoblast is destroyed by the maternal syncytium, or my own that 
the trophoblast is the more active part, remnants of the maternal tissue being, 
however, persistent, as was also noted in many carnivores. 
