102 A. A. W. HUBRECHY. 
able circumstances as far as the conditions of nutrition are 
concerned. It cannot well be doubted that this has been 
very conducive to allowing the central nervous system to 
reach that stage of higher development and complication by 
which the human brain is so widely separated from that of 
other mammals.1 
More than once the mammalian blastocyst, when it actively 
attacks the maternal tissues, reminds one of a temporary 
internal parasite. It is clear that the more perfect the 
arrangements are by which the temporary parasite obtains 
its food from the mother-host, the more intense and perfected 
can be its nutrition and growth during life in utero. 
a. Hedgehog (Hrinaceus).—We will now select a few 
examples out of the numerous cases offered by all the different 
orders of mammals. 
We begin by what may at the same time be looked upon as 
a primitive, as a full-fledged and a very instructive case, viz., 
the hedgehog, in which we find the above-said resemblance 
with the higher Primates, as was admitted by investigators 
of early human blastocysts, such as Siegenbeek van Heu- 
kelom (’98), and Peters (799). 
The blastocyst, at a period when the entoderm is not yet 
clearly separated from the embryonic knob (cf. p. 8), and 
when the cavity inside of the trophoblast is not either very 
spacious, is found in the lumen of the uterus at the bottom 
of a comparatively deep pit which has originated pre- 
paratory to pregnancy by a proliferation of the maternal 
tissue, which was fully described elsewhere by myself (89, 
p. 312), and by Resink (02). We find the trophoblast of 
the very young embryo closely applied against the maternal 
epithelium of the pit; then against the denuded subepithelial 
1 In connection with this I may just call in mind the curious fact that in 
other mammals, who, beside man, share those favourable adaptations of the 
blastocyst (Tarsius being the best known of them), a very unexpected increase 
in the volume of the central nervous system in its very earliest stages is 
noted. ‘This increase has been more fully described and figured by me else- 
where (07, p. 50, figs. <—p). 
