DEVELOPMENT OF THE ALBINO RAT 



37 



as revealed in the summary may perhaps speak for the trust- 

 worthiness of the age data as concerns my material. 



It will be observed that the ova approach the uterine end of 

 the oviduct while in the 2-cell stage; transit through the last 

 portion of the oviduct, where the greater part of the segmentation 

 occurs, being relatively slow. It is hoped that these data, for 

 the accuracy of which I am dependent on reconstructions, may 

 be of service to others who may desire to collect segmentation 

 stages of the albino rat. 



TABLE 3 



*Not the entire length of oviduct was available for reconstruction. 



In order to obtain the volume changes of the ova during transit 

 through the oviduct, beginning with the pronuclear to 8-cell 

 to 11-cell stages, the following procedure was adopted. As has 

 been shown by my figures, reconstructions were made at a mag- 

 nification of 1000 diameters of ova presenting the stage in ques- 

 tion. The sections of my series measure 10 /i in thickness. 

 In order, therefore, to obtain the correct third dimension, it 

 was necessary to use wax plates 10 mm. thick, in actual prac- 

 tice, five superimposed 2 mm. plates. For the majority of the 

 sections of my series this procedure was relatively simple. How- 

 ever, there was usually a question as to the thickness to be 

 ascribed to the first and last section of any given series, since it 

 was evident, both from the appearance of the section, as seen 

 under the magnification used, and the appearance of the model, 

 that the end sections did not measure 10 n in thickness, and it 



