DEVELOPMENT OF THE ALBINO RAT 51 



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 w r orthy 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, 

 X 2CO. 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 /z by approxi- 

 mately 40 M, 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 n 

 by 55 fj. by approximately 45 p. It is evident that this vesicle 



