DEVELOPMENT OF THE ALBINO RAT 83 



of this and older stages, a question which will receive further 

 consideration in following pages. 



In C of figure 27 (rat No. 94, 8 days) the proamniotic cavity 

 forms a continuous, single space. The figure presented is drawn 

 from two sections; its greater portion, to the base of the ecto- 

 placental cone from one section, the ectoplacental cone from 

 another section. The junction of the membranous wall of the 

 vesicle to the base of the ectoplacental cone, in the two sections 

 used for the figure, was superimposed under camera lucida in 

 joining the portions drawn from the two sections. It is be- 

 lieved that the drawing as presented gives correctly dimen- 

 sions and relations of the different parts of this vesicle. The 

 wall of the antimesometrial portion of the single proamniotic 

 cavity is formed by the primary embryonic ectoderm, the cells 

 of which are for the main of irregular columnar shape, with 

 alternately placed nuclei. These cells are in active proliferation, 

 as is evidenced by numerous mitoses. The wall of the meso- 

 metrial end of the proamniotic cavity is formed of a single layer 

 of cells of the extraembryonic ectoderm; these cells are of quite 

 regular shape with nuclei placed in about the same plane. They 

 stain less deeply than do the cells of the primary embryonic 

 ectoderm. In this egg-cylinder (C, fig. 27) the proamniotic 

 cavity does not extend so near the base of the ectoplacental 

 cone as in a number of other preparations in my possession, 

 showing about the same stage of development; in certain of these, 

 the proamniotic cavity extends to near the mesometrial end 

 of the egg-cylinder. 



A more definite characterization of the different parts of the 

 egg vesicle of the albino rat at the stage of development shown 

 in C, figure 27, end of the 8th day, seems desirable, and in doing 

 so I shall use the terminology used by, Sobotta and Widakowich. 

 The vesicle under consideration has reached a length of 0.65 

 mm., and a width of 0.12 mm. Somewhat more than one- 

 fourth of its length consists of ectoplacental cone or Trager. 

 The cavity enclosed is derived from the cavity of the blasto- 

 dermic vesicle with germ disc, the blastocele, and is termed by 

 Sobotta and Widakowich the 'Dottersackhohle' or yolk-sac 

 cavity. This cavity is bounded by a thin structureless mem- 



