382 STUDIES IN GENERAL PHYSIOLOGY 
occurred at the same time as (or earlier than) in the eggs 
from the same culture kept in oxygen. If the minimum 
amount of oxygen necessary for cleavage is present, the 
velocity of the cleavage is a function of the temperature and 
not of the amount of oxygen present. It is important to 
emphasize the fact that lack of oxygen at room temperature 
does not retard cleavage, as does a reduction in the tempera- 
ture. 
Finally, I convinced myself of the fact, through a special 
series of experiments, that prolonged exposure to cold does 
not diminish the power of the egg to divide. I allowed a 
weak current of hydrogen to pass through a gas-chamber 
which remained on ice for four hours. When I then exposed 
the eggs to room temperature and continued to pass the 
same weak current of gas through the chamber, all the eggs 
divided. The majority reached the four-cell stage, and a 
few even the eight-cell stage. Cleavage then ceased. If 
not all the oxygen is driven out, cleavage proportionate to 
the amount of oxygen present still occurs in spite of the 
prolonged cooling. We are, therefore, justified in conclud- 
ing that when all the oxygen which it is possible to remove 
from the Ctenolabrus egg is driven out, no complete cell- 
division can occur. 
The question now arises in how far a division of the 
nucleus is possible in such an egg. At the surface of the 
Ctenolabrus egg a series of visible changes occurs before the 
first cleavage. In the center of the nucleus several droplets 
of a strongly refractive substance collect, which increase in 
number and size, then coalesce, to resolve again into a large 
number of minute droplets just before cell-division. These 
droplets probably play a réle, as we shall see later, in the 
process of cell-division. It is possible, but not proved, that 
their formation is a function of karyokinesis. These 
changes in the strongly refractive substance also occur 
Digitized by Microsoft® 
