DEVELOPMENT OF THE ALBINO RAT 285 
thus the beginning of the fourth day, in a stage in which 16 cells 
up to 32 cells may be enumerated; the oolemma having been 
lost in the 8-cell stage. The data furnished by Melissinos as 
concerns the mouse, are as follows: The 2-cell stage is obtained 
at the end of 24 hours after copulation, the 6-cell stage during 
the first 12 hours of the second day, and the 28-cell stage during 
the second 12 hours of the second day. The ovum is said to 
pass into the uterine horn at the end of the third day after cop- 
ulation, retaining its oolemma. The account of Sobotta seems 
the more reliable. Hensen describes a 2-cell stage in the guinea- 
pig 22 to 24 hours after copulation, and Bischoff records that the 
ovum of the guinea-pig passes into the uterine horn while in the 
8-cell to 16-cell stage, toward the end of the third day. Heape, 
who has described very fully the segmentation stages of the 
mole (Talpa europea) gives no data as to the rate of segmenta- 
tion. In the explanation of the figures presented it may be noted 
that the ova figured, showing 2-cell to 15-cell stages, were taken 
from the oviduct. His figure 20, showing an ovum ‘fully seg- 
mented’ was obtained from the anterior end of the uterus. Asshe- 
ton gives for the rabbit the following data: The 2-cell stage is 
obtained about 24 hours and the 4-cell stage about 26 hours 
after coitus. The third series of divisions begins about 28 hours 
after coitus, so that by the end of the second day a typical morula 
of 16 cells to 20 cells is to be found. Between 73 hours and 96 
hours the beginning of the blastodermic vesicle formation is to 
be noted. Ova obtained 80 hours after coitus, still surrounded 
by the oolemma, were removed from the uterine horn. Data 
as to the relative position of the ova in the oviduct in the several 
stages of development discussed, are given. As concerns the 
sheep, Assheton states that the ova pass into the uterine horn 
early on the third day after mating. The pronuclear stage is 
to be observed the second day, and the first segmentation at the 
end of the second day. By the fourth day, with the ova in the 
8-cell stage, they are found in the upper end of the uterine horn. 
The blastodermic vesicle formation begins with the 16-cell stage. 
Again, according to Assheton, the ova of the pig pass to the uterus 
about the third day after fertilization, if I read him rightly, 
reaching the uterus in the 4-cell stage, although ova in the 2-cell 
