DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS, 441 
immediately in front of the embryo in which mesoblastic 
formation is comparatively retarded. I have already shown 
that a bilaminar area with similar boundaries appears in the 
ova of rats and mice at a period which corresponds closely 
with that during which the pro-amniotic area of the rabbit is 
defined, and thus in the rat and the mouse the inherited ten- 
dency to pro-amnion formation becomes manifest long after its 
supposed precocious expression in the inversion of the layers. 
It is probable, therefore, that the early inversion of the 
germinal layers is not due to acceleration of development, but 
to other causes which have modified more usual processes of 
growth. 
All the ova in which inversion occurs become embedded at 
an early period in a crypt of the uterine mucosa which is soon 
converted into a closed space, and in all the epiblast is covered 
from an early period bya layer of trophoblast. That inclusion 
within a uterine crypt is not, alone, sufficient to cause inversion 
is demonstrated by the development of the hedgehog’s ovum, 
which is embedded in a uterine crypt at an early period, but 
still retains the uninverted condition (24). Two circumstances 
in the development of this animal are especially noteworthy: 
the first is that the growth of the ovum and the expansion of 
the uterine crypt in which it is contained are simultaneous 
and equal events; and the second that as the epiblastic disc ex- 
pands, the trophoblast which covers it becomes thinner. In 
the rat and the mouse the progress of events is very different, 
At a very early period the margins of the trophoblast which 
cover the epiblast appareutly become adherent to the walls of 
the yolk-sac on which they rest (fig.8, Pl. XXIII) ; consequently 
during its subsequent growth the epiblast, which is thus bound 
down to the yolk-sac by the trophoblast, is prevented from ex- 
panding laterally. It increases as an oval mass, and produces 
the invagination of the dorsal wall of the sac (fig. 9, Pl. XXIII). 
The close apposition of the walls of the yolk-sac to the maternal 
tissues prevents the extension of the trophoblast round the 
hypoblast, and its margins become adherent to the uterine 
tissues. As it cannot extend over the yolk-sac, and as its pro- 
