118 L. V. HEILBRUNN 



the moving granules as well as the specific gravity of the fluid through 

 which they move. In order to determine the specific gravity of 

 granules, cells are crushed and the granules are obtained in free sus- 

 pension. The specific gravity of the granules is then determined by 

 centrifuging them in various strengths of sucrose solution of known 

 specific gravity. In a similar way, one can determine the specific 

 gravity of the cells as a whole. Then, if one knows the fraction of the 

 cell occupied by granules, and this can readily be determined, one 

 can calculate by simple algebra the specific gravity of the fluid 

 through which the granules move. As a matter of fact, determina- 

 tion of the specific gravity of granules is not too exact, for it involves 

 the assumption that, in the sugar solution, the granules do not change 

 their specific gravity. The granules probably do change their specific 

 gravity while exposed to the hypertonic sugar solution. However, 

 the error involved is not very great, and presumably it is not so great 

 as the error involved in determining the radius of the granules. For 

 such small objects as cell granules, it is not possible to arrive at an 

 exact value for their diameter. Unfortunately, the value that is 

 used is squared in the Stokes' formula. Another difficulty that lies 

 in the way of exact determinations of absolute viscosity is the fact 

 that the Cunningham correction, which must be applied (see above), 

 is not a very accurate correction. 



All in all, any measurements of the absolute viscosity of proto- 

 plasm that have been obtained are not very accurate. Nevertheless, 

 for the two cases in which measurements of protoplasmic viscosity 

 have been made by both centrifuge and Brownian movement meth- 

 ods, reasonably good agreement has been obtained. These measure- 

 ments of absolute protoplasmic viscosity have shown that the old 

 notion that protoplasm is a very viscous fluid is wrong, at least for 

 some types of protoplasm. Older authors had frequently emphasized 

 the high viscosity of protoplasm, and some indeed had assumed an 

 arbitrary value of 1000 centipoises. Now we know that some types 

 of protoplasm have a viscosity of approximately 3 to 5 centipoises. 



In some types of cells, various types of granules move under the 

 influence of centrifugal force. This is a help to the student of proto- 

 plasmic viscosity. In the egg of the sea urchin Arbacia (see Fig. 2), 

 there are several types of- granules that move centrifugally. Chief 

 among these are the small colorless granules. Some larger red 

 granules, several times as large as the colorless ones, also move cen- 

 trifugally. Because of their larger size, the red granules move more 



