EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 145 
though more complicated, were yet more archaic. The primitive 
earliest stages are unknown to us, and probably meant to 
remain unknown for ever, as so many transition forms that 
must have existed in the paleeozoic epoch. 
The “diffuse” placentation of the Lemurs should be 
looked upon as a second case of a simplified arrangement 
leading to a very similar result, as in the horse, but not 
necessarily, though not impossibly, along the same phylo- 
genetic track. There is no reason why this simplification 
should not have arisen more than once; also in the EKdentata, 
Manis, as has already been expressed above, gives another 
example of it. 
That in Lemurs the evolution of the diffuse placenta has 
been different can be in part made probable by the fact 
of a very curious early phenomenon noticed in Nycticebus. 
We have already described in Insectivores, Rodents, and 
Carnivores the very early and very effective adhesion of the 
youngest blastocysts to the uterine wall, and the phenomena 
of placentation consequent upon this. Nycticebus has feetal 
investments which, in the latter half of the period of pregnancy, 
can, together with the enclosed foetus, be quite easily washed 
out of the maternal crypts, the trophoblastic villi not being 
in any way confluent with maternal tissue. ‘here are two 
intact layers of epithelium between the maternal and the 
embryonic blood (Figs. 146 and 152). We would thus expect 
that the early blastocyst cannot either boast of any strong 
adhesion to the uterine wall, but would agree with the horse, 
pig (Fig. 153), sheep (Fig. 154), ete. Nycticebus, however, 
wholly differs from these latter by the fact that in those early 
stages, when the blastocyst has a diameter of 5—11 mm., it 
is very firmly kept in its place in the uterine horn, in which 
we find it, by another peculiarity. ‘The horn (and the blasto- 
cyst inside of it) have, namely, undergone a quite unusual 
degree of distension ; the median portion of the genital ducts, 
however, is not in any way comprehended in this enlargement. 
Consequently the blastocyst is kept in its place very effectu- 
ally, although there is no surface adhesion whatever, and 
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