386 ARTHUR ROBINSON. 
seventh day. The ovum is 159, long and 61, broad, and 
it is divided into fourteen sections. Fig. 10 represents the 
eighth section. 
The yolk-sac is invaginated upon itself from the proximal 
towards the distal end. Inthe cavity of invagination the epi- 
blast and part of the trophoblast are situated. The nuclei of 
the hypoblast (HY) form a single row. They are placed at 
fairly regular intervals over the trophoblast and epiblast, 
but more irregularly in the remaining portions of the wall of 
the yolk-sac. The hypoblastic protoplasm stains less deeply 
with carmine than the protoplasm of the epiblast and tropho- 
blast. 
The mass of epiblast (Z) is 33 long and 41 4 broad. It 
contains no cavity. It is not distinctly divided into cell areas, 
and it contains many very large nuclei. 
The trophoblast (7') is 106 long and 45 y broad. It also 
is a multinucleated protoplasmic mass. The nuclei are placed 
at varying intervals, and they are of irregular but generally 
large size. 
The distal end of the trophoblast rests against the epiblast, 
from which, however, it is quite distinct. 
Laterally the trophoblast is partially surrounded by the 
invaginated portion of the yolk-sac, and the part of the tropho- 
blast outside the yolk-sac lies in contact with the uterine sub- 
mucosa. Proximally it is free in the cavity of the uterine 
crypt. 
There is a distinct cavity projecting into the interior of the 
trophoblast from its distal end. 
Fig. 11, Pl. XXIII, represents the eighth of a series of seven- 
teen sections of an ovum of a mouse at the end of the seventh or 
the commencement of the eighth day. The ovum is 171 » long 
and 87, broad, It is divided longitudinally, and evidently 
consists of the same three portions recognised in the three pre- 
ceding ova. The yolk-sac is more distinctly invaginated than 
in the preceding stage. It encloses the epiblast (#) and the 
greater part of the trophoblast (7). The hypoblast (HY) 
which forms the yolk-sac wall is thickest over the epiblast and 
