94 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 proamniotic 

 cavity, 0.5 mm., of which 0.2 mm. falls to the antimesometrial 

 portion lined by primary embryonic ectoderm. This egg-cylin- 

 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- 

 metriai 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, consists 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 



