DEVELOPMENT OF THE ALBINO RAT 319 
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- 
| | ex. ect fh = 
| \4) \ 
{ 
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 of each vesicle is slightly folded. % 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 Triger; ect.n., ectodermal node; ez. 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 
