DEVELOPMENT OF THE ALBINO RAT 21D 
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 
