326 G. CARL HUBER 
portion of the proamniotic cavity, developing in the extra- 
embryonic ectoderm, is of greater dimension. Two relatively 
large spaces, bordered by a single layer of cells of the extra- 
embryonic ectoderm, are to be observed. At the junction of 
the extraembryonic ectoderm and the ectodermal vesicle of 
primary embryonic ectoderm a further space of triangular out- 
line may be seen. The primary embryonic ectoderm is ar- 
ranged in the form of an oval-shaped vesicle, forming the anti- 
mesometrial end of the egg-cylinder. Its wall is relatively thin 
at the region of its apposition to the extraembryonic ectoderm, 
just below the triangular space above mentioned. This ecto- 
dermal vesicle is peculiar in that its cavity contains the re- 
mains of four cells. A study of the series of sections shows 
that these cells do not represent the crest of a fold of the wall 
of this vesicle, since they are not nearly so distinct in preceding 
and succeeding sections. It may only be conjectured that 
during the rearrangement of the cells of the ectodermal node, 
resulting in the formation of the ectodermal vesicle, certain of 
the cells became separated from the wall and remained free in 
the cavity. The primary embryonic ectoderm, forming the 
wall of the ectodermal vesicle is readily differentiated from the 
extraembryonic ectoderm, both by the fairly sharp definition of 
the ectodermal vesicle and by reason of the fact that its cells 
stain somewhat more deeply than do the cells of the extraem- 
bryonic ectoderm, as also the cells of the visceral entoderm. 
In the egg-cylinder shown under B of figure 27 (rat No. 96, 8 
days) the antimesometrial portion of the proamniotic cavity, 
developing in the ectodermal node, and the mesometrial portion 
of the proamniotic cavity, developing as several discrete spaces 
in the extraembryonic ectoderm, have in part joined to form 
a single proamniotic cavity. The mesometrial portion of this 
cavity is still bridged by a septum of extraembryonic ectodermal 
cells, closing off a relatively large space found in its mesometrial 
portion. With the junction of the antimesometrial and the 
mesometrial portions of the proamniotic cavity, the primary 
embryonic ectoderm and the extraembryonic ectoderm become 
a continuous layer, the line of union of the two portions, however, 
remains evident and is readily recognized in all the egg-cylinders 
