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. 



