THE INTERPHASE CELL AND THE CYTOPLASM 



proved that fibrous proteins analogous to myosin occur in the cyto- 

 plasm. However, the evidence for cytoplasmic long-chain particles 

 has not rested only on the properties of these 'structure proteins'. 

 PoLLiSTER^^ pointed out that a sub-microscopkal pattern of order may 

 affect the orientation of cell inclusions at the microscopical level. Study 

 of elongated fibroblastic cells in tissue culture supports this suggestion, 

 for both the arrangement and motion of the mitochondria and gran- 

 ules is generally parallel to the axis of a cell process. Elongated fibro- 

 blasts generally show a slight cytoplasmic birefringence (Swann and 

 MiTCHisoN^^). It is not known, however, whether this is due to orienta- 

 tion of the fluid interior of the cell or within the gelated cell wall. 



It has been claimed by Pfeiffer*^ *i that certain physical phenomena 

 characteristic of long-chain particles in solution can be exhibited by 

 living cytoplasm. By forcing isolated portions of cells or of slime-mould 

 Plasmodium through capillary tubes he appears to have demonstrated 

 both streaming birefringence due to molecular orientation by shearing 

 forces, and a high viscosity which varies with the rate of flow. Such 

 experiments, however, do not necessarily reveal properties of fluid 

 endoplasm, for a new external gel may form round isolated cytoplasm 

 or even more serious changes may occur. Kamiya*^ says: 'it is impos- 

 sible to make slime mould protoplasm flow through a glass capillary 

 without fatal results'. 



Estimations of viscosity 



The evidence from measurements and estimates of endoplasmic 

 viscosity in various types of living cell is by no means uniform. Although 

 the contents of some diflferentiated cells apparently are gelated or 

 highly viscous (Chambers and Renyi,*^ Seifriz,** Renyi,*^ Speidel,*^), 

 the cytoplasm of a number of marine eggs stratifies in the centrifuge 

 at rates which correspond to viscosities of only a few times that of 

 water (Heilbrunn*'), Moreover, Howard*^ has shown that the 

 viscosity of the internal cytoplasm in the unfertilized sea-urchin egg is 

 independent of the rate of shear, and thus behaves as a true liquid. 

 She observed the stratification of the granules of the egg when centri- 

 fuged at different speeds, and found that when allowance was made 

 for the opposing tendency of these inclusions to return to their normal 

 random distribution, the rate of movement was directly proportional 

 to the force applied. 



One method which has been used to estimate protoplasmic viscosity 

 has been to introduce magnetic particles into the cytoplasm and to 

 observe them in motion within the cell under the influence of external 

 magnetic fields. This method was first used by Heilbronn*^ in a study 

 of the Plasmodium of a slime mould, and has been applied by Crick 

 and Hughes^'' to cells in tissue culture, by taking account of the 



c 25 



