400 STUDIES IN GENERAL PHYSIOLOGY 
a word on the importance of comparative methods in physi- 
ology. If we had confined our experiments to the Cteno- 
labrus egg, a generalization of the facts observed would 
have been as follows: Cleavage is impossible without oxygen. 
Had we confined our experiments to the Fundulus egg, we 
should have come to the opposite conclusion. In reality, 
conditions are such that in some forms a cleavage is possible 
without oxygen, while in others it is impossible. The same 
may be said regarding protoplasmic motion. I do not as yet 
consider it as settled that every muscle is able to do a large 
amount of work without free oxygen. 
IX. THE EFFECT OF THE REMOVAL OF OXYGEN ON THE 
SEGMENTATION OF SEA-URCHIN EGGS 
If freshly fertilized sea-urchin eggs are introduced into 
a gas-chamber and a strong current of hydrogen is sent 
through it, one cleavage always occurs, and sometimes two. 
If, however, before beginning the actual experiment, all of 
the oxygen necessary for cleavage is driven out of the eggs 
and the gas-chamber (by placing the latter upon ice for two 
hours and sending a current of hydrogen through it), no 
cleavage occurs, even though we wait from three to four 
hours. If after this the eggs are again exposed to air, 
cleavage begins in about forty to fifty minutes. But all the 
eggs first divide into two cells, and only a few divide at once 
into three or four cells. The number of the latter is not 
greater in the experimental eggs than in the normal eggs of 
the same culture. Such phenomena are very probably 
attributable to polyspermia. These facts show that in sea- 
urchin eggs neither a division of the cell nor of the nucleus 
is possible without oxygen. In this particular they behave 
like the eggs of Ctenolabrus. We must now raise the ques- 
tion: Is the inability of cleavage in sea-urchin eggs also 
the consequence of molecular changes which are brought 
about by lack of oxygen? This, indeed, seems to be the case. 
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