VISCOSITY CHANGES DURING MITOSIS 445 



SUMMARY 



1. The centrifuge method of measuring protoplasmic vis- 

 cosity is a perfectly sound method from a physical standpoint. 

 With it quantitative data may be obtained. 



2. The velocity of granular movement under the influence of 

 centrifugal force was taken as a measure of the viscosity. 



3. The viscosity of Cumingia egg protoplasm was plotted for 

 the period between fertilization and first cleavage. The data 

 may be presented in the form of a curve (fig. 1). 



4. In both maturation divisions and cleavage, appearance of 

 the spindle is always preceded by a sharp viscosity increase and 

 followed by a sharp viscosity decrease. 



5. At the conclusion of mitosis, within a minute or two before 

 the division of the cell is completed, a sharp increase in viscosity 

 occurs. . 



6. When two mitoses follow each other, the concluding vis- 

 cosity increase of the first mitosis becomes the initial viscosity 

 increase of the second. 



7. In Nereis, the viscosity changes between fertihzation and 

 cleavage are similar to those found in Cumingia. Apparently 

 these changes are not as marked as in Cumingia, a fact which 

 is correlated with the larger size of the Nereis egg and the rel- 

 atively small size of its spindle. 



8. The discussion includes a critique of the microdissection 

 method of viscosity measurement as well as certain interpretative 

 considerations. 



