DEVELOPMENT OF THE ALBINO RAT oul 
considered. The antimesometrial portion of this vesicle, its roof, 
consists of a single layer of somewhat flattened cells, the parietal 
or transitory ectoderm. The parietal ectoderm presents on its 
inner surface a few—four in the section figured—entodermal 
cells of irregular outline. These may be designated, after So- 
botta, as cells of the parietal entoderm. 
Vesicle B, of figure 24, taken from the same rat as was vesicle 
A (rat No. 46, 6 days, 14 hours) presents a very favorably cut 
vesicle, which, however, is slightly compressed from side to side, 
so that its form appears more nearly circular in the sections cut 
in the plane of the figure, than were they cut at right angles to 
this plane. This is especially true of the ectoplacental cone, 
which for the greater part appears in only two sections of 10 » 
thickness, while in the plane of the figure it measures nearly 90 u. 
Cognizance of this is to be taken in considering the relative 
size of the ectoplacental cone as shown in this figure. This 
vesicle is only very slightly older than that shown in A of this 
figure. Its ectoplacental cone is made up of a core of relatively 
large cells, bordered by more flattened cells, which in this prepara- 
tion stain somewhat more deeply than do the more centrally 
placed cells. These covering cells are continuous with the cells 
of the parietal ectoderm. The cell mass projecting into the 
blastocele is more definitely circumscribed than in the slightly 
younger stage shown in A of this figure. The ectodermal node 
appears as an oval mass composed of compactly arranged cells, 
and is separable on all sides from the surrounding cells. The 
yolk entoderm, which may now be known as the visceral layer 
of the entoderm (Sobotta) passes as a single layer of cells of quite 
regularly cubic or short columnar form, nearly about the ecto- 
dermal node to reach the base of the ectoplacental cone, extend- 
ing over on the parietal ectoderm at one side (see right side of 
figure). A few of the cells of the parietal entoderm, three in 
the figure, are evident. The parietal ectoderm forming the roof 
or antimesometrial portion of this vesicle consists of a single layer 
of flattened cells, which rest on, and are adherent to the decidual 
tissue; the uterine epithelium lining the decidual crypt in which 
the vesicle is lodged having in part disappeared in the immediate 
region of the vesicle. 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 2 
