DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 427 
sida an increasingly early and complete separation of the ovum 
from all sources whence maternal nutriment might be derived 
has possibly occurred, and consequently there has been in- 
creased yolk formation and general increase in the size of the 
ovum; but in the descent from the “ Protamniota” to the 
Mammalia the ova have been placed in conditions progressively 
more favorable to the direct nutrition of the developing germ 
by the maternal tissues, and the ovum has therefore diminished 
in size. 
4, One of the peculiar features of all ova with a large or 
comparatively large amount of yolk, e. g. the ova of the Elas- 
mobranchs, Teleosteans, Lacertilians, and Aves, is the great 
preponderance of the hypoblast over the epiblast. The 
latter portion of the ovum in the very early stages is merely a 
small disc, which rests upon one pole of a large hypoblastic 
mass. It is only as development proceeds that the margins 
of the epiblastic disc extend until the mass of hypoblast is 
completely enclosed, as in the Elasmobranchii, the Teleostei, 
and the Lacertilia, or almost completely enclosed as in Aves; 
and we have no reason for supposing that in the large-yolked 
protamniotic ova any great variation from this arrangement 
prevailed. 
5. In the developing ova of all the Vertebrata below the 
Protamniota two cavities appear at different periods: the 
first, the segmentation cavity, lies between the epiblast and 
hypoblast ; it is transitory in character, and disappears coin- 
cidently with the formation and extension of the second 
cavity—the archenteron—which appears within the hypo- 
blast. 
The segmentation cavity is in some cases very small, and it 
may be that this peculiarity was at first associated with rapid 
yolk formation occurring during the segmentation period, part 
of the yolk being deposited in the situation that would other- 
wise have been occupied by the segmentation cavity. 
As the phenomena here referred to are met with in all the 
Vertebrata below the Protamniota, there is every probability 
that the tendency to their production was transmitted to the 
