EARLY ONTOGENETIG PHENOMENA IN MAMMALS. 107 
to placentation, which have here been alluded to, we find the 
same in very diverse form among Insectivora, Rodentia, Car- 
nivora, and others, and it would lead us too far to give a 
detailed account of all the possible variations known up to now. 
We can already conclude from a comparison of the hedgehog 
and certain Primates that the trophoblast’s specific modifica- 
tions are all the more considerable the more extensive the 
surface is over which the blastocyst comes into contact with 
and fixes itself in the maternal tissue. ‘Tarsius, which had a 
comparatively limited surface of attachment, retains an un- 
modified trophoblast over a very considerable portion of the 
growing blastocyst. 
c. Rodentia and Carnivora.—Among Rodents we simi- 
larly notice that besides cases in which the rapidly-enlarging 
blastocyst is only very partially attached to the mucosa (as is, 
for example, the case along a hoof-shaped part of the surface 
close to the embryonic shield in the rabbit), there are others 
in which the (generally comparatively much smaller) blasto- 
cysts disappear partially or wholly in the maternal tissue, and 
become in the latter case enclosed in a decidua capsularis, there 
being valid grounds for looking upon this latter process as 
the more primitive. The mouse, Arvicola, the guinea-pig are 
examples of this. Selenka (’83, 784), Duval (’87), Jenkinson 
(02), Disse (06), and others have given detailed descriptions 
of the very considerable modifications which the trophoblast 
cells undergo after the blastocyst has become definitely lodged 
in the maternal subepithelial tissue. They increase very 
considerably in size, become confluent for the formation of a 
syncytium, contribute towards the formation of spacious 
lacune for the reception of maternal blood in the immediate 
vicinity of the developing blastocyst; in short, they are of 
great significance for the welfare of the young embryo. In 
many Rodents the trophoblastic proliferation assumes a 
different character according to the part of the surface which 
we examine, F'. Muller (07), and in those where the embryo 
becomes wholly surrounded by maternal tissue the tropho- 
blast does not necessarily behave as it does in the hedgehog, 
