DEVELOPMENT OF THE ALBINO RAT 29 



4-cell stage. The material includes the oviducts of two rats, 

 Nos. 50 and 63, killed 3 days and 1 hour after the beginning 

 of insemination, with twelve ova in the 4-cell stage. In figure 

 11, there are shown two views of each of the models obtained 

 by reconstruction after the Born method, at a magnification of 

 1000, of the three 4-cell stages found in the oviducts of rat 

 No. 50. The drawing of the reconstructions do not present the 

 conventional figures of the 4-cell stage of the mammalian egg. 

 In none of the twelve ova of this stage was the plane of section 

 such as to include all of the four cells in one section. Nearly all 



Fig. 12 Cross-section of right oviduct of rat No. 50, 3 days, 1 hour. X 100. 

 This section contains two cells of a 4-cell stage of the albino rat, slightly com- 

 pressed between the folds of the tubal mucosa. 



lie in a portion of the tube which presents a relatively narrow 

 lumen, and appear as if slightly compressed between the folds of 

 the mucosa. I am not disposed to regard this as a resultant of 

 fixation, due to contraction at the time of fixation. In figure 

 12 is reproduced a cross section of the right oviduct of rat No. 

 50, passing through a 4-cell stage. It is evident that in shape 

 the two cells included in the section, conform in the main to 

 the form of the lumen, the mucosa appearing as slightly retracted 

 to one side of the egg mass. This conformity in shape of cell 

 mass to the form of the lumen I find quite general in my material 

 showing segmentation stages of the albino rat, to some extent 



