338 G. CARL HUBER 
The egg-cylinder shown in figure 29 presents a total length 
of 1.15 mm., a width of approximately 0.18 mm. The ectopla- 
cental cone presents a length of 0.4 mm. and of the proamniotie 
cavity, 0.6 mm., of which 0.2 mm. falls to the antimesometrial 
portion lined by primary embryonic ectoderm. This egg-eylin- 
der differs only in shape and size from that shown in C of figure 
27, obtained 8 days after insemination. The primary embryonic 
and extraembryonic ectoderm lining or enclosing the proam- 
niotic cavity are readily differentiated. The primary embryonic 
ectoderm, derived from the ectodermal node, constitutes a pseu- 
dostratified epithelium, composed of relatively long columnar 
cells, with nuclei radially placed with reference to the lumen of 
the proamniotic cavity, and shows active cell division, no less 
than 12 mitotic figures occurring in the section figured. The 
protoplasm of its cells stains distinctly deeper than does that 
of the cells of the extraembryonic ectoderm. The cells of the 
latter are of cubic, short columnar, or polyhedral shape, ar- 
ranged in a single or double layer, with no definite arrangement 
of the long axes of its nuclei. It is, therefore, possible readily 
to distinguish—by reason of shape and size of cells, relative posi- 
tion of nuclei, reaction to stain of protoplasm—between the 
cells of the primary embryonic and extraembryonic ectoderm, 
and to determine the sharp line of Junction at which the two 
types of cells form a continuous layer, a fact which will receive 
further consideration in dealing with the anlage of the mesoderm 
as observed in slightly more advanced stages. At the meso- 
metrial end of the proamniotic cavity, the cells of the extraem- 
bryonic ectoderm become continuous with the cells at the base 
of the ectoplacental cone; in the region of this junction, active 
mitosis are often to be observed. In this egg-cylinder the visceral 
entoderm may readily be differentiated into two portions. The 
portion which surrounds the primary embryonic ectoderm to 
nearly the region of its junction with the extraembryonic ecto- 
derm, censists of a single layer of broad, flattened cells which 
assume a cubic or short columnar shape as the mesometrial 
border of the primary embryonic ectoderm is approached. 
This portion of the visceral entoderm we have designated as 
