DEVELOPMENT OF THE ALBINO RAT 309 
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 
Stas 
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by 
Fig. 24 Sections of blastodermic vesicles or blastocysts of the albino rat 
showing the early stages of entypy of the germ disc. % 200. <A and B, rat No. 
46, 6 days, 14 hours; C, rat No. 54, 6 days, 16 hours; ect.pl., ectoplacental cone or 
Triiger; 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 afew hours, it is evident 
