DEVELOPMENT OF THE ALBINO RAT 39 



tends through a relatively long period and into the beginning of 

 the second day, by which time the ova have migrated about one- 

 fourth of the length of the oviduct, there occur only three suc- 

 cessive mitotic divisions, including the first segmentation divi- 

 sion, namely mitoses resulting in 2-cell, 4-cell and 8-cell stages 

 while the ova are in transit in the oviduct. In making this 

 statement it is assumed that in the successive segmentations, the 

 several cells divide synchronously, which is not in conformity 

 with the fact. These three mitotic divisions are spaced at in- 

 tervals of about 18 hours. In the next following division, the 

 fourth, the ovum passes from the oviduct to the uterine horn. 

 Since the normal gestation period of the non-lactating albino 

 rat is only 21 to 23 days, this slow rate of increase in volume and 

 multiplication of cells during the first four days of development 

 is of especial interest and is very probably to be accounted for 

 by the inadequacy of the food supply of the ovum during its 

 transit through the oviduct. 



The presence or absence of the oolemma has not been considered 

 in discussing the segmentation stages of the albino rat. In my 

 own material, the oolemma was clearly observed in certain of 

 the 2-cell stages, but not in the 4-cell nor 8-cell stages. Wida- 

 kowich reports that he has observed in the albino rat, loss of 

 the oolemma even in the 2-cell stage. Since all of the material 

 covering these stages was fixed in Carnoy's fluid, a fluid with a 

 relatively large glacial acetic acid content, it may be questioned 

 as to whether the fixative used may not be in part responsible 

 for the early disappearance of the oolemma, though neither 

 Hubrecht nor Sobotta considers the presence or absence of an 

 acid in the conserving fluid of special moment in the fixation of 

 the oolemma. Sobotta finds that the oolemma disappears in 

 the ova of mice during the 8-cell stage. The early disappear- 

 ance of the oolemma in the albino rat may be offered as an ex- 

 planation of the fact that the egg mass during segmentation and 

 transit through the oviduct does not, as a rule, present a spherical 

 form but appears compressed and molded to fit the form of the 

 lumen. A similar explanation is offered by Sobotta to account 

 for the irregularity of form assumed by the ovum of the mouse 

 after loss of the oolemma. In the forms in which the oolemma 



