STUDIES IN MAMMALIAN EMBRYOLOGY. 339 
of quite as much comparison with the early stages of the hedge- 
hog’s blastocyst (which, like man’s, is not free in the uterus 
lumen, but lodged in the recess formed by the decidua reflexa), 
as that it does with those of the pig’s. And I am at a loss to 
compare the arrangement of pig and hedgehog with each other. 
The first is visibly an adaptation by which an increased surface 
attachment of the blastocyst brings about more copious 
nutrition, the embryo being lodged in the uterus lumen; the 
latter is an adaptation by which the blastocyst that was in 
the earliest stages sunk into the maternal mucosa, and removed 
from the lumen, directly receives maternal blood into the 
thickness of its outer wall, the special arrangement by which 
this is brought about being also in the hedgehog, as in man, 
gradually restricted to a discoid region, and no longer occupy- 
ing the whole of the spherical surface. 
These few remarks may suffice to point out the relative 
significance that I wish to attach to these early stages of the 
hedgehog’s blastocyst. The manner in which the early blastocyst 
leaves its place at the bottom of the decidual depression, and 
after resorption of the uterine epithelium, becomes lodged in 
the centre of the decidual proliferation, was described above 
(pp. 317, 318), as was also the gradual development of free 
spaces in its outer wall, and of widened blood-cavities with 
proliferating endothelium in the surrounding maternal tissue 
(p. 319). The fact that the maternal blood passes at this very 
early stage from the latter into the former is of the very highest 
importance. ‘This fact alone will henceforth oblige us to sub- 
stitute in the phylogeny of the human placenta arrangements 
as we find them in the hedgehog to those that have hitherto 
been adhered to, and that were especially derived from the 
pig. At the same time the Erinacean arrangement cannot 
either be derived from that of the pig, and so we shall have 
to look for the preceding genetic stages amongst the Didelphia 
and amongst other lowly organised Monodelphia. 
That, indeed, at a very early stage of the hedgehog’s blasto- 
cyst the lacune (sp.) that bathe its outer surface, and of which 
we have studied the development in preceding chapters, are 
