DEVELOPMENT OF THE ALBINO RAT 279 
This series, it seems to me, corroborates the statement previously 
made, that the detail of form of the living segmenting ova of 
certain mammals, while in transit through the oviduct, is in a 
great measure dependent on the configuration presented by the 
lumen of the oviduct in the particular region in which they are 
found. ; 
12-cell to 16-cell stages. Rat No. 51, killed 4 days after the 
beginning of insemination, presents the end of the segmentation 
stages in the oviduct. In the genital tract of this animal there 
were found eight morula masses, five on the left side and three on 
c D 
Fig. 19 Sections of morula stages of the albino rat. % 200. A, 8-cell stage, 
rat No. 57, 3 days, 17 hours; six of the eight cells, each cut in the plane of its 
nucleus, are included in the section figured. B,C, and D, 12-cell to 16-cell stages, 
from right oviduct, rat No. 51, 4 days. 
the right side. Itis somewhat difficult to determine definitely 
the number of cells constituting each of the morula. The number 
appears to vary between 12 and 18, though nearly all of the moru- 
la masses-show certain nuclei in mitosis. The left oviduct with 
a short adjoining segment of the uterine horn was reconstructed. 
Slight tension was applied to the tissue prior to fixation, which 
accounts for the elongation of the proximal loop of the oviduct. 
The model is reproduced in figure 18. As is evident on study 
of this figure, three of the morula masses are situated in a por- 
tion of the oviduct just prior to its insertion in the uterine tube. 
These are closely grouped between folds of the mucosa. <A 
fourth morula is found in the uppermost part of the uterine 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 2 
