DEVELOPMENT OF THE ALKI.YO RAT 



21 



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. 



SEG A L E N TATION STAGES 



2-cell stage. The material on which my own observations of 

 this stage are based is listed in table 2. 



TABLE 2 



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 Burckhard, 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 



