DEVELOPMENT OF THE ALBINO RAT 35 



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 



Fig. 19 Sections of morula stages of the albino rat. X 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. It is 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 



