338 
SAMUEL RITTENHOUSE 
The third cleavage plane is equatorial and divides the egg into 
eight equal blastomeres ; four of which are situated at the upper 
pole and four at the lower pole of the egg, as seen in fig. 9. This 
is the condition when segmentation is regular, and may be de- 
scribed as two four-celled stages of half size superimposed one 
upon the other and then the upper set rotated to the left. While 
the formation of the eight-celled stage was always nearly the same 
in the eggs that I followed, after the division was completed, the 
blastomeres did not always retain the same relative positions. 
Sometimes there occurred a separation of the cells at one side of 
the equatorial furrow and the blastomeres rolled apart in such a 
manner as to form a curved sheet. In others this separation and 
unrolling of the blastomeres was less definite and the final arrange- 
ment was such as is shown in fig. 10 
The irregularity in the relative position of the blastomeres 
begin with the eight -celled stage and is more or less characteristic 
of all later stages up to the formation of the blastula. While 
there is diversity of arrangement of the blastomeres, nevertheless 
I am led to believe that the division of the individual cells is reg- 
ular and takes place just as though the blastomeres always held 
the same relative position. 
The fourth segmentation follows after a short period of time. 
Fig. 11 shows a sixteen-celled stage which is nearly regular, but 
the cleavage cavity has already been formed within the mass of 
blastomeres and they are thus pushed away from the center of the 
egg. In this stage the cell lineage can be traced even in the forms 
that are somewhat irregular. But in the older stages the arrange- 
ment of the cells is more irregular and, owing to the opacity of the 
egg, it is difficult to follow with accuracy the descent of the cells. 
Fig. 12 shows a later stage in which the arrangement of the cells is 
more irregular than is common in eggs of the same age. 
As stated before, the divisions follow each other at short inter- 
vals. Within two hours after the eggs were laid they had under- 
gone the processes of maturation and fertilization, and had passed 
beyond the sixty-four-celled stage. The cells continue to divide 
with the same rapidity, while within them the cleavage cavity 
is also gradually enlarging. Fig. 13 shows a stage in which the 
