DEVELOPMENT OF THE ALBINO RAT 



75 



in his later communications. Vesicle A, figure 25, when com- 

 pared with vesicle C of figure 24, shows only a slight difference in 

 degree of development. Vesicle A is of more elongated and of 

 more distinctly cylindrical form. Its thin-walled portion (an- 



Fig. 25 Longitudinal sections of blastodermic vesicles of the albino rat, show- 

 ing entypy or inversion of germ layers with early stages in egg-cylinder formation. 

 The ectoplacental cone of each is not cut through its entire length and the lower 

 portion ot each vesicle is slightly folded. X 200. A, B, and C, rat No. 35, 8 

 days, 18 hours, after insemination. To fit properly into the entire series these 

 three vesicles should be from the early hours of the seventh day after insemina- 

 tion. ect.pl., ectoplacental cone or Trager; ect.n., ectodermal node; ex. ect., 

 extraembryonic ectoderm, early stage of its ingrowth shown in vesicle A; p. ect., 

 parietal or transitory ectoderm; v.ent., visceral layer of entoderm; p.ent., cells 

 of parietal entoderm. 



timesometrial portion) is longer, its cavity more extensive; 

 this is owing to a further flattening of the cells of the parietal 

 or transitory ectoderm. In vesicle A in the section preceding 

 the one figured, the ectoplacental cone is thicker by about two 



