DEVELOPMENT OF THE ALBINO RAT 



65 



vesicles are cut in an oblique plane, which may deviate only 

 a little from the longitudinal or may approach a cross axis, 

 while only a relatively small number of vesicles were cut favor- 

 ably in the longitudinal plane, and the majority of these are in 

 series cut parallel to the plane of the mesometrium. The vesi- 

 cles on which the special consideration of this stage is based are 

 reproduced as seen in sections, in figure 24. 



Vesicle A, of figure 24 (rat No. 46, 6 days, 14 hours), is drawn 

 from two successive sections. The upper portion of the figure 



Fig. 24 Sections of blastodermic vesicles or blastocysts of the albino rat 

 showing the early stages of entypy of the germ disc. X 200. A and B, rat No. 

 46, 6 days, 14 hours; C, rat No. 54, 6 days, 16 hours; ect.pl., ectoplacental cone or 

 Trager; ect.n., ectodermal node; p.ect., parietal or transitory ectoderm; v. ent., 

 visceral layer of entoderm; p.ent., parietal entoderm. 



was drawn under camera lucida from one section, then by super- 

 imposing certain of the cells so as to give proper orientation, the 

 lower half of the figure was added from the succeeding section. 

 The slightly oblique plane in which this vesicle was cut made this 

 procedure desirable. This relatively small vesicle seems in 

 excellent state of fixation, as is evident from the symmetrical 

 outline shown by the successive sections of the series. When 

 compared with vesicle E of figure 23, though the two are sepa- 

 rated in time of development by only a few hours, it is evident 



