430 ARTHUR ROBINSON. 
have indications that a tendency to the production of certain 
phenomena, which appear in other classes of the Vertebrata, 
has been transmitted through the Protamniota to the Mam- 
malia, where, although it is exhibited upon comparatively 
small ova, it produces, under relatively similar circumstances, 
very similar effects to those met with in the larger ova of 
various other Vertebrates. 
The hypothesis of the nature of the various component 
parts of the young mammalian blastocyst, which has been 
stated in the preceding pages, agrees in so far as the ento- 
dermic nature of a large portion of the blastocyst wall is con- 
cerned with that which was proposed by Minot in 1885, and 
which was again restated by him in the ‘ American Naturalist’ 
in 1889; but I cannot see any facts which support Minot’s 
suppositions, that the whole of the subzonal epithelium is ento- 
dermic, and that therefore there is a complete inversion of 
the germinal layers in the young developing mammalian ovum ; 
that the whole of the inner mass in the mammalian blastocyst 
is ectoderm, and that the blastocyst cavity is comparable with 
the segmentation cavity of the lower Vertebrates. On the 
contrary, the evidence we possess appears to me rather to 
point to the foliowing conclusions : 
(1) That the greater part of the subzonal epithelium is 
entodermic, but a small portion is ectodermic. 
(2) That the main part of the inner mass of the mammalian 
blastocyst is ectodermic, but asmall portion of it is entodermic. 
(3) That the cavity of the mammalian blastocyst does not 
correspond with the segmentation cavity of the lower Verte- 
brates, but with the archenteron. 
The Formation of the Celom. 
In most accounts of mammalian development the formation 
of the coelomic cavity is very shortly and incompletely dealt 
with, and little can be gathered from published descriptions 
beyond the fact that the space appears within the mesoblast. 
We do not know, however, except in the cases of a few animals, 
whether it appears in the embryonic or extra-embryonic area, 
