THE INTERPHASE CELL AND THE CYTOPLASM 



The particle at first rotates rapidly; but the resistance offered by its 

 surroundings increases and the early rapid phase of rotation gives place 

 to a slower one in which movement then involves the breakdown of 

 protoplasmic structure by shearing (Figure 6). When the field is 

 switched off, there is at once an elastic recoil which is small in compari- 

 son with the original deflection. The endoplasm of cells in tissue cultures 

 is thus a weakly elastic fluid. It also shows another property char- 

 acteristic of some colloids, known as thixotropy, which is an increase 

 in fluidity during mechanical agitation. During repeated movement of 

 a magnetic particle within a cell, the deflection due to a magnetic 

 field of constant strength was shown to increase (Figure 7). It has 

 already been shown qualitatively by Chambers^^ that the protoplasm 



130 



120 



1W 



90 



a 



A 



Tf 



70- I 



60 



50 - 



W 



off 



on' 



off 



160 ^ 

 150 ^ 



no 



130 

 120 



no ^ 



100 

 ■90 

 80 

 70 



Seconds 



Figure 8 Angular displacement of a magnetic particle in a solution 



of sodium oleate when magnetic fields of a 3 oersteds, b 24 oersteds 



were applied for the intervals indicated. From Crick and Hughes*" 



{By courtesy t Academic Press Inc., New York). 



of an Amoeba tends to liquefy on mechanical agitation with a micro- 

 dissection needle, 



A further experiment of Crick and Hughes suggests that the physical 

 properties of the cytoplasm of fibroblastic cells is not necessarily due to 

 an ultra-microscopic structure of long molecules, for if magnetic par- 

 ticles are suspended in a highly elastic liquid, such as a solution of 

 sodium oleate, and are rotated as in the experiments on the living cell, 

 the recoil which results when the magnetic field is switched off is then 

 almost as large as the initial deflection (Figure 8). This contrast in 

 the magnitude of the elastic recoil in the two instances suggests that the 



27 



