DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 3875 
other wall of the cavity, the lower, is formed by a mass of 
cells (HY) of larger size and more irregular shape. 
This ovum measures in its greatest diameter 57 yu, and in its 
least diameter 46 uw. Its long axis is at right angles to the 
long axis of the uterine canal. 
It is apparently a little younger than the mouse ovum 
figured by Selenka (44, in Taf. i, fig. 1), and about the same 
age (five days) as that represented by Duval (9, pl. i, 
fig. 73). 
Fig. 2, Pl. XXIII, represents a section of another ovum from 
the same uterus as the ovum depicted in fig. 3. It is vesicular, 
and measures 79 pu in its longest diameter, which lies at right 
angles to the long axis of the uterus, and 41 yin its shortest 
diameter. The ovum is divided into eight sections, and the 
fifth of these is represented by fig. 2. The cells forming the 
upper wall of the cavity are flatter than the cells in the same 
position in the ovum represented in fig. 38, Pl. XXIII, and the 
floor of the cavity is formed over part of its extent by only a 
single layer of large cells. The blastodermic cavity is con- 
siderably larger than in the other ovum from the same uterus, 
Fig. 4, Pl. XXIII, represents the sixth section of a mouse 
ovum, which has been divided into eleven sections. The long 
diameter of this ovum, which lies in the long axis of the uterine 
canal, measures 49 yu, and its short diameter is 26. It is at 
about the end of the fifth or the commencement of the sixth 
day of development, and corresponds with the ovum repre- 
sented in Taf. i, fig. 1, by Selenka (44), and in pl. i, fig. 76, 
by Duval (9). It is a nucleated protoplasmic vesicle in which 
no cell outlines are distinguishable. The roof of the vesicle 
(ED) is formed by a layer of protoplasm, which contains a 
single row of nuclei, the long axes of which are parallel with 
the surface of the vesicle. The floor of the vesicle is a mass 
of granular protoplasm (HY), containing many nuclei which 
are larger and more rounded than those in the roof. The 
protoplasm of the roof stains with carmine more deeply than 
that of the floor. 
A section of another mouse ovum at the sixth day is repre- 
