DEVELOPMENT OF THE ALBINO RAT 291 
broad diameter—that is, in plane of sections, and since it passes 
through seven sections of 10 thickness, measures approximately 
70 w inits third dimension. It consists of 24 cells, as estimated 
by counting the nuclei contained in its several sections. The 
cells vary in size as well asin shape. The nuclei are for the main 
of spherical form, presenting one or several large nucleoli and 
fine chromatin granules. This morula is found within a fold 
of the mucosa, each side of which presents a slight depression, 
lined by slightly flattened epithelium. T his morula: mass lies 
Fig. 20 Sections of morula mass and early stages of blastodermic vesicle of 
the albino rat. > 200. A, B, C, rat No. &2, 4 days and 15 hours. D and E, rat 
No. 68. 4 days and 16 hours. A, late stage of morula; B, shows the very be- 
ginning of the formation of the segmentation cavity; C, D, E, early stages of 
blastodermic vesicle, in E, a distinct covering layer in the thicker portion or 
floor of the vesicle is evident. 
free in the lumen of the mucosal fold, and not in contact with the 
uterine epithelium. The outline and extent of the shallow 
depressions found in the opposing walls of the mucosal folds 
conform to shape and size of the morula mass contained, which 
appear as if slightly retracted as a result of fixation. 
In B, figure 20, is figured one of the sections of a series passing 
through a morula stage, comprising as estimated 30 cells and 
measuring 90 u, by 60 uw, by approximately 50 », in which the 
very beginning of the formation of the segmentation cavity is 
shown. Near one pole the outermost cells have separated slightly 
from the more deeply placed cells, so that an irregularly shaped 
