DEVELOPMENT OF THE ALBINO RAT 293 
toward the mesometrial border of the uterine horn, while its 
thinner portion is designated as its roof, directed toward the 
antimesometrial border. Therefore, in slightly older stages 
than thus far figured, the vesicle lies with its long axis approxi- 
mately at right angle to the long axis of the uterine horn. In 
further description of the blastodermic vesicle, I shall use the 
term ‘floor’ and ‘roof’ as here specified. In D and E of figure 20, 
there are reproduced typical sections of the two blastodermic 
vesicles taken from the uterus of rat No. 68, killed 4 days, 16 
hours after insemination. Vesicle D measures 90 » by 30 u 
by approximately 60 u, and is of distinctly ovoid form and 
slightly compressed. This vesicle is found lying free in a long 
but narrow fold of the mucosa, both sides of which are slightly 
molded in conformity with the form of the vesicle. The long 
axis of the vesicle is still parallel to the long axis of the uterine 
horn. The roof of the vesicle appears as if slightly contracted, 
though when traced through the series of six sections it does 
not appear folded. The roof is composed of only a few cells, 
perhaps seven in all. The segmentation cavity presents a regu- 
lar outline. This vesicle supports the contention of Widakowich, 
that the form of the blastodermic vesicle of the rat is dependent 
in a measure on the form of the space in which it is found. Vesi- 
cle E, of figure 20, measuring 85 uw by 45 »w by approximately 
40 uw, presents a roof that is slightly folded and shows an early 
stage in segmentation cavity formation. A figure of the vesicle 
is included since it represents more clearly than any other blas- 
todermic vesicle of the albino rat in my possession, a differentia- 
tion of a layer of surface cells in the mass constituting its floor. 
This is a question to be more fully considered in further discussion. 
In all the measurements of blastodermic vesicles thus far 
given, even in those given for the morula mass shown in A, figure 
20, it is evident that one of the short diameters is appreciably 
shorter than the other. The vesicles are not only of ovoid form, 
but slightly flattened, so that even when not folded, the form of 
the vesicle as seen in section, even when cut parallel to the long 
axis of the respective vesicles, is dependent in a measure on the 
plane of the section, whether parallel to the longer or the shorter 
