VISCOSITY CHANGES DURING MITOSIS 443 



occurring in the sea-urchin egg. The latter perhaps presents a 

 somewhat simpler case, for the first cleavage division does not 

 immediately follow the maturation divisions. The viscosity 

 changes that occur in the sea-urchin egg are probably typical 

 of mitosis in general. There is a marked viscosity increase in 

 early prophase, then a decrease, and finally an increase just 

 before the cell divides. This final viscosity increase in the sea- 

 urchin egg continues as the early prophase viscosity increase 

 of the second cleavage division. In Cumingia and Nereis the 

 maturation divisions are related to each other and to the first 

 cleavage division as the first cleavage division is related to the 

 second in the sea-urchin egg. Apparently whenever two mitoses 

 follow each other in rapid succession, the viscosity increase at 

 the conclusion of the first provides the essential viscosity in- 

 crease for the beginning of the second. 



In the various eggs studied the viscosity changes are appar- 

 ently not of the same magnitude. In Nereis they are not as 

 great as in the other eggs. This is readily understood in the 

 light of the fact that the Nereis egg is considerably larger and 

 its mitotic spindle forms a much smaller fraction of the material 

 of the egg. 



The magnitude of the changes has been determined only for 

 Cumingia. In this egg the viscosity rises from an arbitrary 

 value of 2 or 3 to a value of 16, and then drops to about its orig- 

 inal value. These changes are very rapid and the increase 

 in viscosity just mentioned occurs within a minute or two. Such 

 a sharp and sudden viscosity increase is almost conclusive evi- 

 dence that a gelation or coagulation has occurred within the 

 egg. This does not mean that the entire protoplasm has gelled. 

 Presumably only a part is affected. It seems likely that were 

 the entire protoplasmic mass of the egg to gel, the viscosity 

 increase would be greater. It is more logical to assume, however, 

 that only that portion, of the cytoplasm which is concerned with 

 spindle formation undergoes gelation. This is borne out by the 

 fact that in the Nereis egg, in which the spindle forms a smaller 

 fraction of the egg material, the viscosity change is not so great. 



The mechanism of viscosity increase and decrease or gelation 

 and solation within the cell is not yet understood. It might be 



