IV. VISCOSITY MEASUREMENTS 119 



rapidly through the cell, pushing their way through both the proto- 

 plasmic granules and the smaller granules in their path. The speed 

 of these larger granules can be used for a determination of the viscos- 

 ity of the protoplasmic suspension (fluid plus smaller granules). The 

 value obtained for this viscosity is several times greater than the 

 viscosity value for the fluid through which all the granules m6ve. 

 This is what one would expect from our knowledge of the viscosity of 

 suspensions. 



Fig. 2. A centrifuged sea urchin egg (lo). The egg 

 is ahnost readj' to break into two halves. 



The fact that there may be several types of granules moving 

 through a cell is a help also in determinations of relative viscosity. 

 During the mitotic division of a cell, the viscosity of the protoplasm 

 changes markedly; it becomes more viscous before the appearance 

 of the mitotic spindle and then becomes rapidly less viscous. It might 

 be argued that these changes as observed by the centrifuge method 

 are not real and that the greater or lesser speed of the granules moving 

 through the cell is due to changes in the specific gravity of the gran- 

 ules. For, according to Stokes' law, if the granules that move centrif- 

 ugally w^ere to become heavier, they would move more rapidly. The 

 fact that the movement of all types of granules, heavy and light alike, 

 agree in showing the same types of viscositj^ change is strong evi- 



