DEVELOPMENT OF THE ALBINO RAT 295 
shown in all 35 nuclei, two of which are in a late diaster phase. 
By excluding the nuclei that appear to be cut in two, appearing 
thus in two successive sections, I estimate that the blastodermic 
vesicle is made up of only about 30 to 32 cells. 
In figure 22, there are reproduced typical sections of four 
blastodermic vesicles taken from the uterus of rat No. 53, killed 
five days after the beginning of insemination. This uterus con- 
tained in all, eight vesicles, one of which was distinctly pathologic. 
The four vesicles selected for reproduction and discussion pre- 
sent each certain characteristics worthy of consideration. Vesi- 
cle A, which presents an early stage of segmentation cavity forma- 
Fig. 22 Sections of early stages of blastodermic vesicle of the albino rat, 
xX 200. Rat No. 53, 5 days. 
tion is of interest owing to the number of mitotic divisions it 
contains. As a rule, I have noticed only a few mitoses at this 
stage. In this vesicle, which measures 90 » by 55 » by approxi- 
mately 40 u, there are no less than five mitoses to be noted, three 
of which are in cells forming the roof of the small segmentation 
cavity, and are included in the section figured. This is the only 
vesicle in my possession in which an increase in the size of the 
segmentation cavity is accompanied by active mitoses in the 
cells bounding it. The vesicle lies free in the uterine lumen, 
one wall of which is only slightly pitted. In B of figure 22 is 
reproduced a section of a blastodermic vesicle, measuring 90 u 
by 55 uw by approximately 45 uw. It is evident that this vesicle 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 2 
