376 ARTHUR ROBINSON. 
sented in fig. 5, Pl. XXIII. This ovum also lies free in the 
uterine canal. Its long axis measures 125, and lies parallel with 
the long axis of the uterine canal. Its short axis is 26 mu long. 
The ovum is cut in its long axis, but somewhat obliquely, 
so that the total length is not fully represented in any 
one section, but the sixth out of a series of nine sections 
has the greatest length, and fig. 5 is a representation of that 
section. 
In this ovum both the floor and the roof of the cavity are 
more extensive than in the mouse ovum previously described 
(compare figs. 4and 5, Pl. XXIII). In the ovum represented in 
fig. 5, however, the nuclei of the floor are comparatively very 
large, and the protoplasm surrounding them is more or less 
distinctly marked out into cell areas; but, as in the younger 
ovum (fig. 4), the protoplasm of the floor is less granular, 
and it stains less deeply with carmine than the protoplasm of 
the roof (see fig. 5). 
The youngest mouse ovum described by Duval (9, pl. 1, 
figs. 78 and 74) is a blastodermic vesicle of the fifth day, 
which consists, according to his description, of two parts, an 
outer formed by a single layer of epiblast-cells, on the inner 
surface of which, at the proximal pole of the ovum (thatis, the 
pole lying next the mesometrial side of the uterus), is a small 
mass of hypoblast. During the fifth day the epiblast of the 
proximal pole of the ovum is said to proliferate, and as the 
spherical outline of the vesicle is maintained the thickened 
epiblast projects into the interior of the vesicle, pushing the 
hypoblast before it. The youngest mouse ovum described by 
Selenka has attained this stage (44, Taf. i, fig. 1) ; and he dis- 
tinguishes its component cells into two groups,—an outer layer 
of flattened cells, which he calls Reichert’s membrane ; and an 
inner mass, which he subdivides into a proximal portion or 
epiblast, and a distal portion or hypoblast. 
Fraser’s description of an ovum at six days twelve hours 
(13) corresponds very closely with that given by Selenka, 
except that he calls all the external layer of epiblast Rauber’s 
cell layer, whilst Selenka restricts the term “ Rauber’s cells” 
