VISCOSITY CHANGES DURING MITOSIS 439 



viscosity. This increase is of considerable importance and can 

 of course be studied experimentally in the same manner that 

 the viscosity increase in early prophase has been studied. This 

 I propose to do in the near future. 



The viscosity changes in Arbacia and Cumingia are absolutely 

 parallel. In each case the appearance of a spindle is preceded 

 by an increase in viscosity and followed by a decrease in vis- 

 cosity.' And in both eggs division of the cell is immediately 

 preceded by a viscosity increase. 



Besides the two eggs already mentioned the egg of Nereis 

 was also studied. The Heteronereis form of Nereis limbata 

 is common at Woods Hole. Its early development is well known 

 through the studies of E. B. Wilson and of F. R. Lillie. The 

 egg is immature at fertilization and three complete mitoses 

 occur between fertilization and cleavage. In Nereis viscosity 

 changes are not as sharply marked as in Cumingia or Arbacia 

 and the egg is therefore more difficult to study. The Nereis 

 egg is much larger than the eggs of the other two forms studied 

 and it contains a considerable amount of yolk material. In 

 proportion to the size of the egg, the spindle is small. We might 

 expect that the viscosity changes associated with the appearance 

 of the spindle would be masked in an egg when the spindle con- 

 stitutes but a small fraction of the material of the egg. 



A number of tests were made on the Nereis egg, but only one 

 series is included here, these are given in table 13. All tests 

 were made at a uniform centrifugal speed. The new-model 

 Bausch & Lomb centrifuge was used and the handle was turned 

 five times in five seconds. 



The table shows that in the immature Nereis egg the proto- 

 plasm is very stiff. Soon after fertilization and the disappear- 

 ance of the germinal vesicle the viscosity decreases. Following 

 this there are four times of viscosity increase, at 20, 35, 47, and 

 65 minutes after fertilization. These viscosity increases pre- 

 sumably correspond with, 1) early prophase of first maturation 

 division; 2) conclusion of first and early prophase of second 

 maturation division; 3) conclusion of second maturation divi- 

 sion and early prophase of cleavage mitosis; 4) conclusion of 



