48 G. CARL HI BER 



cavity, eccentrically placed and passing clearly through two 

 of a series of five sections of 10 /* thickness, is evident. The small 

 cleftlike cavity is bounded by the surrounding cells, the outline 

 of which is distinct. So far as may be judged from the appear- 

 ance noted as presented in the two sections in which this cavity 

 is found, this arose as a single space and as a result of the separa- 

 tion of the enclosing cells. 



In C, of figure 20, there is presented a slightly older stage 

 showing the blastodermic vesicle formation and measuring 80 /z, 

 by 50 ju, by approximately 50 n, comprising as is estimated, 34 

 to 36 cells. Unfortunately, the lower part of this vesicle is 

 slightly folded as is shown in the lower left of the figure. The 

 appearances presented in the sections are reproduced as faith- 

 fully as could be. Owing to the folding, a portion of the thin 

 wall is cut tangential ly. The more darkly colored curved line 

 represents in reality the outer boundary of this portion of the 

 vesicle. The segmentation cavity in this vesicle is distinctly 

 larger than that shown in B of this figure. In the section re- 

 produced the segmentation cavity is bounded for the greater 

 part by four somewhat flattened cells, the increase in the size 

 of the cavity being accompanied, it would seem, by a flattening 

 of the enclosing cells. 



In these three closely approximated stages, which, since 

 they are taken from the same uterus are probably separated in 

 time of development by only short intervals, the cells though 

 varying in size and shape, show no essential or fundamental 

 difference in structure, neither in cytoplasm nor nuclei; nor do 

 they show any regularity in arrangement. Only few mitotic 

 figures are to be observed; none in the morula mass shown in A, 

 and but two in each of the other two stages, show r n in B and C. 

 Judging from these preparations, one would be led to con- 

 clude that segmentation cavity formation in the albino rat is 

 not associated nor accompanied by active cell proliferation. This 

 point will be referred to again after the presentation of further 

 material at hand. In slightly older stages of the blastodermic 

 vesicle than here considered, the thicker portion of the vesicle 

 is designated by Sobotta and others as its floor, which is directed 



