DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 887 
the trophoblast, thinnest in the remainder of its extent, where 
it lies in contact with the uterine tissues. 
The hypoblastic protoplasm still stains more faintly than 
that of the epiblast and trophoblast. On the left side of the 
figure, at the point marked with an asterisk, there is an indica- 
tion of a constriction between the epiblast and trophoblast, and 
at this point the hypoblastic protoplasm is aggregated, and con- 
tains two large nuclei. 
The epiblast (£) is 53 long and 49, broad. it contains 
a small oval cavity which is situated nearer the proximal than 
the distal end. The nuclei of the epiblast are large, and the 
majority of them are arranged parallel with and near to the 
periphery of the mass. 
The greater part of the trophoblast (7) is contained within 
the cavity formed by the invagination of the yolk-sac, and the 
margins of the invagination cavity appear to be contracted 
round the trophoblast, which is constricted as it passes from 
the interior to the exterior of the cavity. The distal end of 
the trophoblast lies in close contact with the epiblast. That 
portion of the trophoblast which lies outside the yolk-sac is 
attached by its surface to the uterine tissues, but its proximal 
end hes free in the uterine crypt. 
Fig. 12, Pl. XXIII, represents the seventh section of a rat’s 
ovum, which is divided longitudinally into thirteen sections. 
The ovum is about the same age as that of the mouse last de- 
scribed—that is, it is about at the end of the seventh or the 
commencement of the eighth day. The greatest length of 
the ovum is 209 u, and its greatest breadth 91 p. 
A detailed description of this ovum is unnecessary because 
its general features correspond with those of the mouse ovum 
last described, but there are some special points which require 
attention. 
The invaginated portion of the yolk-sac is distinctly con- 
stricted round the region where the epiblast and trophoblast 
lie in contact with each other, and in this region the hypo- 
blast is thicker than elsewhere; indeed, on the right-hand 
side of the figure it is seen to contain a double row of nuclei. 
