DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 425 
of a segmentation cavity which remains more or less open to 
the exterior, and which may contain one or more large cells 
(46, Taf. xvii, fig. 8). Apparently this cavity remains and 
becomes the cavity of the blastocyst; the cell or cells in the 
interior and those at the margin of the opening in the wall of 
the cavity proliferate, and thus the vesicle becomes very similar 
in appearance to the blastocyst of the bat (46, Taf. xxiii, 
fig. 6; Taf. xvi, fig. 11), except that the greater portion of the 
wall is formed of columnar instead of flattened cells; but the 
vesicle in both cases consists of an outer wall to which the 
inner mass is adherent, and the outer wall is incomplete over 
the centre of the inner mass, where a depression, called the 
blastopore, is noticeable. In this case also there is no abso- 
lute proof that the columnar cells which form the greater part 
of the blastocyst wall are any more closely related to the cells 
which cover the inner mass than they are to the cells which 
form the margin of the latter part of the ovum—that is, there 
is no more proof that they are epiblastic than that they are 
hypoblastic, and at a more advanced stage after the cells of 
the monodermic portion of the blastocyst wall have become 
flattened (46, Taf. xviii, fig. 3), and the extension of a second 
layer over the primitively monodermic wall has commenced, it 
is absolutely impossible to assert with any degree of certainty 
which of the two layers of the didermic area is the one under- 
going extension, for in the marginal portion of the didermic 
region the cells of both layers are flattened, and on the right- 
hand side of the figure the appearance seems to be, if any- 
thing, more in favour of the extension of the outer layer over 
the inner than the inner over the outer. 
As in all the Vertebrata except the Mammalia, the archen- 
teric space appears within the hypoblast ; and as in the rat, the 
mouse, and the hedgehog amongst the mammals, the blasto- 
cyst cavity, which in all probability is merely a modification 
and extension of the archenteric cavity, appears in a situation 
similar to that oceupied by the archenteron of the lower Verte- 
brates, it seems improbable that in other mammals a cavity 
which bears similar relations to the embryo should be bounded, 
