118 A. A. W. HUBRECHT. 
thinner—form of placenta than that of the Carnivora, but at 
the same time to approach most closely to that type, whereas 
amongst the Insectivora, Sorex provides us with an example 
(Hubrecht, 7944, Fig. 74) of a yet more extensive prolifera- 
tion of the maternal uterine epithelium before the allan- 
toidean attachment of the blastocyst comes about than 
even Perameles. At all events, the placentation of Perameles, 
characterised by so intimate a fusion between foetal and 
maternal elements, should never be classed amongst those 
forms of placenta which are either primarily primitive (as 
yet unknown to us) or secondarily simplified (Ungulates, 
Lemurs, Cetacea, etc.). 
Cuaprer V.—Dirrerent Aspects AND Derralns oF 
PLACENTATION. 
1. Embryonic (Trophoblastic) and Maternal (Tro- 
phospongian) Preparatory Processes.—We have in the 
preceding chapter followed the mammalian blastocyst in its 
very varied attempts to remain attached to the wall of the 
maternal mucosa; and we have seen that either part of or— 
all the trophoblast-cells bring 
about this fixation by peculiar modifications. We once find 
in some exceptional cases 
the blastocyst attached either by its surface diametrically 
opposite the embryonic shield (Tarsius) or by the surface 
contiguous to the embryonic shield (Lepus, bats, mole, 
Perameles among Didelphia) or by both these surfaces 
together (catarrhine monkeys). Or, again, the blastocyst 
may be fixed in a zonary or ring-like shape, and the axis of 
this ring may be perpendicular to and below the embryonic 
shield (Sorex), or it may run parallel to the shield (Carni- 
vores), or finally a double batch of proliferating trophoblast 
may be present, not, as in the catarrhine monkeys, above 
and below the developing embryo, but right and left of it 
(Tupaja). Again, the blastocyst may be fully enclosed in 
maternal tissue, and the trophoblastic activity may then 
