No. 2.] THE ORIGIN OF THE SEX-CELLS. 217 
excepting that these two sets of blastomeres do not lie directly 
opposite each other, but have undergone a slight rotation. 
These cells flatten down and fit together at the center, the 
cell a! of the primary blastomere A coming directly in contact 
with the cell 4! of the primary blastomere & (PI. II, Fig. 26). 
When viewed from the opposite side we would find a reversed 
condition, the primary blastomeres A and & forming the con- 
nection with one another. We here see the “cross furrow,” 
which is so often found in eggs at this period of segmentation. 
In one half hour after this cleavage begins, the complete four- 
celled stage is reached. 
The third cleavage commences about one hour and a quarter 
after the appearance of the first cleavage. Each blastomere 
divides separately, the cleavage, as before, starting in the center 
and running centrifugally. There is evidently a decided 
rotation to the left at this stage. In fifteen minutes from the 
beginning of this cleavage we have eight cells formed; there 
are, however, as yet five in one group, and three in the other; 
gradually one sinks down, and we have the complete eight- 
celled stage, in which there are two groups of four cells lying 
directly across one another. While the cells of this stage 
usually assume this position, there is considerable variation 
found at this period. 
As before, without a period of rest, the fourth cleavage 
appears and runs centrifugally to form the sixteen-celled stage. 
This stage is reached about an hour and forty minutes from 
the appearance of the first cleavage plane. When viewed 
from the pole shown by Pl. II, Fig. 29, we find that the four 
cells occupying the most prominent position are derivatives 
of cell B, Fig. 24. 
The cells become so small, and are so opaque, that desis 
impossible to follow the cleavage further. This also seems 
unnecessary, since it appears to be total and equal ; and thus 
impossible to trace the formation of organs to any especial cell. 
In those forms which Metschnikoff has studied, he notes 
that the centripetal and centrifugal methods of cleavage remain 
constant for the different species, and that in this way the 
different eggs could be distinguished, even if contained in the 
