DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 3881 
mass in the blastodermic cavity of the mouse, not even the 
hypoblastic mass which Duval describes ; and when the inner 
cell mass does appear it is purely epiblastic, not partly 
epiblastic and partly hypoblastic, as is the case in the 
rabbit, according to the description of Kdélliker (27) and 
Rauber (88). 
During the latter part of the sixth day the remains of the 
blastodermic cavity are entirely obliterated, yet the ovum 
retains a vesicular form; but the cavity in its interior at the 
end of the sixth day is the cavity of the yolk-sac, which has 
now become more distinctly developed. 
Fig. 8, Pl. XXIII, represents a section of an ovum found ina 
series of vertical transverse sections of the uterus. The ovum 
is at about the commencement of the seventh day of develop- 
ment, and corresponds to the ova figured by Selenka (44, Taf. 
1, fig. 7) and by Duval (9, pl. i, fig. 83). 
It has been cut longitudinally. It is 121 m long, and 64 
broad. It consists principally of a large hypoblastic sac, 
upon the proximal end of which rests a conical cap of ecto- 
derm (£ D). 
Comparison of figs. 6, 7, and 8, Pl. XXIII, shows that the 
large yolk-sac in fig. 8 has been produced by the vacuolation 
of the multinucleated mass of protoplasm which formed the 
floor of the blastodermic vesicle, and the rudiment of the 
hypoblast. The vacuolation commences during the sixth day 
in the periphery of the proximal portion of the hypoblast, and 
from thence it extends throughout the whole mass, converting 
it into asac. The cavity of the sac is traversed by nucleated 
protoplasmic strands, which are the remains of the central 
portion of the original mass. 
The proximal and distal walls of the yolk-sac are compara- 
tively thick, especially the proximal, which is most intimately 
associated with protoplasmic strands of the interior. The 
lateral walls are thin, and they lie in contact with the walls 
of the crypt in which the ovum is contained. The epithelial 
lining of the uterus has disappeared from certain portions of 
the area with which the walls of the yolk-sac are in contact, 
