98 G. CARL HUBER 



several sections drawn in figure 30 is approximately indicated 

 by the several crosses found to the left of the egg cylinder drawn 

 in figure 29. 



In A of figure 30, there may be observed a vacuolization of the 

 protoplasm of the more peripherally placed cells of the ecto- 

 placental cone, the vacuoles enclosing maternal blood cells. The 

 more centrally placed cells of this ectoplacental cone show a 

 tendency to concentric arrangement. Figures B and C present 

 structural appearances nearly identical. The egg-cylinder is 

 bounded by the thin layer of parietal or transitory ectoderm having 

 scattered masses of nucleated protoplasm on its inner surface. 

 This membrane of apparently homogeneous structure stains 

 sharply in well fixed preparations and may be readily discerned. 

 The cells of the visceral entoderm, somewhat taller in the section 

 taken nearer the antimesometrial pole (C), present clearly the 

 three zones to which attention has been drawn. The cells of 

 the extraembryonic ectoderm bounding the mesometrial portion 

 of the proamniotic cavity, are of cubic, short columnar, or poly- 

 hedral form disposed in single or double layer, presenting relative- 

 ly lightly staining protoplasm. In D of figure 30, the cells form- 

 ing the primary embryonic ectoderm are of distinct columnar 

 shape, with relatively deeply staining protoplasm and nuclei 

 arranged nearly in a single layer except for such as show mitotic 

 phases. The cells of the primary embryonic entoderm are of 

 a broad, pavement type for a greater part of the circumference, 

 and may be contrasted with the cells of the visceral entoderm 

 shown in B and C of the figure; the latter are absorptive cells, the 

 former not. This series of figures, more especially B, G, and D, 

 show clearly the absence of bilaterality in the egg-cylinders of 

 the albino rat at this stage of development. The slight com- 

 pression observed in this egg-cylinder, as shown in the figures, 

 I regard as not of moment. 



Fig. 31 Longitudinal sagittal section of egg-cylinder of the albino rat show- 

 ing anlage of the mesoderm. X 200. Rat Xo. 34, 8 days, 18 hours, after insem- 

 ination; ect.pl., ectoplacental cone or Trager; p.ect., parietal or transitory 

 ectoderm; pr.emb.ect., primary embryonic ectoderm; ex.ect., extraembryonic 

 ectoderm; pr.emb. ent., primary embryonic entoderm; mes., mesoderm in anlage; 

 pr.c, proamniotic cavity; v.ent., visceral entoderm. 



