DEVELOPMENT OF THE ALBINO RAT aoe 
embryonic ectoderm and the extraembryonic ectoderm meet. 
The special cytomorphosis undergone by the columnar cells 
of the sides of the egg-cylinder, in contradistinction to those of 
the antimesometrial end, will be considered in later pages. The 
visceral layer of the entoderm extends to the base of the ecto- 
placental cone, in part passing over onto the layer of parietal 
ectoderm. In the section figured, cells of the parietal layer of 
the entoderm are not evident. The ectoplacental cone has 
grown in length in the direction of the lumen of the uterus or the 
mesometrial border. In the great majority of my preparations 
this structure is slightly compressed from side to side, so as to 
be broader in a plane parallel to the long axis of the uterus. In 
vesicle A, it is cut at right angles to the long axis of the uterus, 
thus appears as much narrower than in the other two vesicles 
of figure 26, which were cut in a plane parallel to the plane of 
the mesometrium. The increase in size of the ectoplacental 
cone is the result of active cell proliferation. Mitotic figures 
to the number of one, two or three, may now be observed in 
nearly every section of this structure. The parietal or transitory 
ectoderm, continuous with the base of the ectoplacental cone, 
has been reduced by this stage to a thin, practically homogeneous 
membrane, presenting scattered, flattened nucleated cells on its 
inner surface. This thin membrane is now quite firmly adherent 
to the wall of the decidual crypt, throughout nearly its whole 
extent. 
Under B of figure 26 (rat No. 81, 7 days, 22 hours) there is 
shown a representative section of a vesicle which is slightly more 
advanced in development than that shown in A of this figure. 
The antimesometrial portion of the proamniotic cavity, the 
anlage of which was shown in the preceding stage, is well estab- 
lished. Its wall, consisting of primary embryonic ectoderm 
is composed of a single layer of cells with nuclei in essentially 
the same plane. The primary embryonic ectoderm forms a 
closed vesicle (Ectodermblase, Selenka) distinctly demarked 
from the extraembryonic ectoderm. In this as in the preceding 
stage the extraembryonic ectoderm forms a long cylindrical 
structure continuous at its mesometrial end with the base of the 
