DEVELOPMENT OF THE ALBINO RAT 265 
therefore, that in the albino rat, 24 hours after the beginning of 
insemination, the ova are to be found in the pronuclear stage, 
with the ova distributed in the end of the third to the beginning 
of the fourth major loop of the oviduct, a portion of the oviduct 
having a relatively wide lumen and lined by a much folded 
mucosa and possessing a relatively thin muscular wall, having 
thus migrated about one-fourth of the length of the oviduct. 
SEGMENTATION STAGES 
2-cell stage. The material on which my own observations of 
this stage are based is listed in table 2. 
TABLE 2 
ee ea ap er i ee 
RECORD NUMBER | aes as ee pena NUMBER OF OVA STAGE OF DEVELOPMENT 
60 | 1 day, 18 hours | vi 2-cell stage 
59 | 2 days 8 2-cell stage 
| iS 7, 2-cell stages; 
58 | 2 days, 17 hours 8 ere ee 
| \ 1, 3-cell stage 
61 | 2 days, 18 hours 8 | 2-cell stage 
| | 10, 2-cell stages; 
62 | 2 days, 22 hours 11 J ; Bes; 
ii 1, 3-cell stage 
Thus in all 40 ova after the completion of the first segmenta- 
tion division and 2 ova in the 3-cell stage, in each of which the 
undivided blastomere presents a nucleus in mitosis. 
My own material lacks stages showing the formation of the 
first segmentation spindle, the conjugation of the two pronuclei, 
and the first segmentation division. I am forced to proceed 
from the pronuclear stage to that showing the first two blasto- 
meres. It was not possible to supplement my material after this 
was sectioned and the stages determined, since it was only after 
leaving The Wistar Institute that this gap in my series was recog- 
nized. This is the more to be regretted since neither Melissinos, 
Sobotta and Burekhard, nor Kirkham and Burr, all of whom 
have considered maturation and fertilization as observed in the 
albino rat, discuss these stages in their account. In the albino 
rat, the fusion of the two pronuclei on the first segmentation 
