418 ARTHUR ROBINSON. 
spreading epiblast: its situation, however, is marked, at least 
for a time, by the adherence of the hypoblast and epiblast (24). 
In such cases the maternal fluids must pass through both 
layers before they can distend the cavity of the yolk-sac and 
bring about the extension of the nutritive area ; and apparently 
to facilitate the passage the epiblast soon becomes permeated 
by maternal blood, which circulates through spaces in the epi- 
blastic layer. 
In another respect also there is a close resemblance between 
the ova of rats and mice and those of Lacertilians, for at 
an early period all these ova consist of a small area of epiblast 
resting upon the upper pole of a large yolk-sac; the mesial 
portion of the epiblastic area is separated from the underlying 
hypoblast by a distinct space, which may be looked upon as a 
remnant of the segmentation cavity ; the middle portion of the 
epiblast is thick, but its margins are thin, and, as in reptiles, 
they lie in direct contact with the hypoblast. The fact that 
the hypoblast in the rat and the mouse does not become 
entirely surrounded by epiblast as in the Lacertilia seems to be 
entirely due to close apposition of the hypoblast and the 
decidua in the former animals. The hedgehog’s ovum differs 
from the lacertilian ovum in the opposite direction, for in it 
the yolk blastopore becomes closed at a comparatively early 
period, and the two primitive layers are for a time almost 
entirely separated from each other by a distinct space. Still, 
in the rat, the mouse, and the hedgehog, in spite of the modi- 
fications brought about by the reduction of the hypoblast and 
the inclusion in maternal decidua, the general resemblance to 
the lacertilian ovum is retained, and it is probably due to 
inherited tendencies transmitted both to the Lacertilia and 
Mammalia from a common ancestor, one of the protamniotic 
Vertebrates. ' 
Further, in the rat, mouse, and hedgehog, as in lizards, 
tortoises, birds, and all the lower Vertebrates, a space appears 
within the hypoblastic mass. Van Beneden asserts that this 
is merely a space in the yolk substance, a “lecithopore,” not 
comparable with the archenteron of other Vertebrates. The 
