VISCOSITY 



213 



to displace them will vary directly with the viscosity of the 

 liquid. 



The eggs of fish or echinoderms when centrifuged show strati- 

 fication of the protoplasmic granules (Fig. 108). The number 

 of turns of the centrifuge handle per unit of time required to 

 stratify the egg granules is a measure of the comparative viscosity 

 of the protoplasm. Values are calculated with the aid of Stokes' 

 formula. Runnstrom finds the viscosity of the echinoderm 

 egg to be eight hundred times 

 that of water as determined by 

 centrifuging; this is almost 

 exactly the viscosity of glycerin. 

 Much lower and higher values 

 have been obtained by other 

 workers using the same method 

 on other material. 



The centrifuge method has the 

 advantage of mathematical treat- 

 ment. Its limitations are that it 

 cannot differentiate between 

 localized regions of a cell ; that is 

 to say, it gives a value of the cell 

 as a whole, and no cell is of 

 uniform consistency throughout. 

 There is also the possibility that 

 a force three hundred to five 

 thousand times that of gravity 

 may cause injury, with a result- 

 ing change in consistency. Weber believes that centrifuging 

 produces no injury to protoplasm, yet Nemec has demonstrated 

 that it does considerable damage to cells that are dividing. 

 Perhaps both workers are correct, and the disagreement in 

 their results is due to the kind of cell and the physiological 

 state of the protoplasm. The cells that Weber used were 

 the vegetative ones of the alga Spirogyra, which are probably 

 less sensitive than the eggs used by Nemec, because the 

 latter were in the process of division, a time when an ovum 

 is certainly very sensitive to mechanical disturbances. D. 

 Kostoff has induced various abnormalities in mitosis and even 

 breakage of the chromosomes by centrifuging. 



Fig. 108. — A centrifuged Arbacia 

 egg showing stratification of the 

 protoplasmic granules; the pale, 

 centrally located bodies are yolk- 

 spheres; the black bodies massed 

 at the one (upper) pole are oil 

 globules; the dark bodies at the 

 opposite (lower) polar region 

 are mitochondria. (From E. B. 

 Wilson.) 



