DEVELOPMENT OF THE ALBINO RAT 47 



broad diameter that is, in plane of sections, and since it passes 

 through seven sections of 10 thickness, measures approximately 

 70 M in its 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 as in 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. This morula mass lies 



D 



Fig. 20 Sections of morula mass and early stages of blastodermic vesicle of 

 the albino rat. X 200. A, B, C, rat No. c2, 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 /z, by 60 /z, 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 



