DEVELOPMENT OF THE ALBINO RAT Bak 
of this and older stages, a question which will receive further 
consideration in following pages. 
In C of figure 27 (rat No. 94,-8 days) the proamniotic cavity 
forms a continuous, single space. The figure presented is drawn 
from two sections; its greater portion, to the base of the ecto- 
placental cone from one section, the ectoplacental cone from 
another section. The junction of the membranous wall of the 
vesicle to the base of the ectoplacental cone, in the two sections 
used for the figure, was superimposed under camera lucida in 
joining the portions drawn from the two sections. It is be- 
lieved that the drawing as presented gives correctly dimen- 
sions and relations of the different parts of this vesicle. The 
wall of the antimesometrial portion of the single proamniotic 
cavity is formed by the primary embryonic ectoderm, the cells 
of which are for the main of irregular columnar shape, with 
alternately placed nuclei. These cells are in active proliferation, 
as is evidenced by numerous mitoses. The wall of the meso- 
metrial end of the proamniotic cavity is formed of a single layer 
of cells of the extraembryonic ectoderm; these cells are of quite 
regular shape with nuclei placed in about the same plane. They 
stain less deeply than do the cells of the primary embryonic 
ectoderm. In this egg-cylinder (C, fig. 27) the proamniotic 
cavity does not extend so near the base of the ectoplacental 
cone as in a number of other preparations in my possession, 
showing about the same stage of development; in certain of these, 
the proamniotic cavity extends to near the mesometrial end 
of the egg-cylinder. 
A more definite characterization of the different parts of the 
egg vesicle of the albino rat at the stage of development shown 
in C, figure 27, end of the 8th day, seems desirable, and in doing 
so I shall use the terminology used by Sobotta and Widakowich. 
The vesicle under consideration has reached a length of 0.65 
mm., and a width of 0.12 mm. Somewhat more than one- 
fourth of its length consists of ectoplacental cone or Trager. 
The cavity enclosed is derived from the cavity of the blasto- 
dermic vesicle with germ disc, the blastocele, and is termed by 
Sobotta and Widakowich the ‘Dottersackhéhle’ or yolk-sac 
cavity. This cavity is bounded by a thin structureless mem- 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 2 
