DEVELOPMENT OF THE ALBINO RAT 281 
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 
DISTANCE RELATIVE 
RECORD SIDE RECON- AGE NUMBER STAG LENGTH OF OF OVA LENGTH OF 
NUMBER STRUCTED OF OVA ack OVIDUCT FROM FIM- TUBE 
BRIA TRAVERSED 
cm. cm, 
106 R 1 day 8 pronuclear 3.2 0.8 0.25 
59 R 2 days 4 2-cell 220i 1.4 0.61 
62 L 2 days, 5 2-cell 2 .45* 7A) 0.82 
22 hrs. 
50 R 3 days, 4 4-cell 2.8 205 0.90 
Il Jatie 
51 L 4 days 5 12- to 16- 2.86 2.86 1.00 
cell 
*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 uw 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 u« in thickness, and it 
