DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 379 
thick protoplasmic floor of the blastodermic vesicle of the 
earlier stages (compare figs. 4 and 5 with fig. 6, P]. XXIII). 
The lateral walls of the vesicle consist of a thin layer of 
nucleated protoplasm, which is more granular and stains more 
deeply with carmine than the protoplasm of the distal pole. 
It clearly resembles the protoplasmic roof of younger vesicles 
(compare figs. 4 and 6, Pl. XXIII). 
The proximal wall (P) of the vesicle is thicker than the 
lateral walls, to which it is otherwise similar, and it is con- 
tinuous with a mass of nucleated protoplasm which occupies 
the interior of the vesicle. This mass of protoplasm is very 
granular, it stains comparatively deeply with carmine, it con- 
tains many large nuclei, and it is indistinctly divided into cell 
areas. The optical characters of the protoplasm of the inner 
mass are similar to those of the lateral and proximal walls, 
but the nuclei of the latter are much smaller than those of the 
inner mass. 
In the interior of the inner mass is a small cavity, from 
which a dark line extends to the surface of the proximal pole 
of the ovum. 
It seems probable that, during the sixth day, the thin roof 
of the blastodermic vesicle is invaginated into the blastodermic 
cavity ; that its nuclei enlarge and proliferate; and that the 
sides of the invagination cavity are gradually approximated 
(figs. 8, 4, 5, and 6, Pl. XXIII). The thick floor of the 
blastodermic vesicle of the earlier stages (figs. 3, 4, 5, and 6, 
Pl. XXIII) becomes the distal pole of the vesicle at a later 
stage. It does not become divided into an outer layer and an 
inner mass, for the inner mass is undoubtedly formed by the 
invagination of the thin roof of the vesicle. 
Fig. 7, Pl. XXIII, is a representation of the tenth section of 
an ovum from the same uterus as the ovum shown in fig. 6, but 
it is more advanced than the latter. It is divided longitudi- 
nally, and also slightly obliquely, so that none of the fourteen 
sections represent the full length of the ovum. 
The section depicted in fig. 7 is 64 u long (from proximal 
to distal end), and 76 w broad. 
