DEVELOPMENT OF THE ALBINO RAT 283 
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 
