THE CENTRAL PROBLEMS OF THE BIOCHEMISTRY OF CELL DIVISION 489 



mental property of mitosis in the living cell. The mitotic apparatus cannot 

 be observed as an organized structure in the cell when it is not dividing, 

 though certain parts of it concerned with the centrioles may be present. 

 It can be said to appear when it is "needed" and to disappear when its 

 work is done. One way of saying this is that the intracellular conditions 

 permit its stability during the period of division and no longer do so when 

 division is completed. This loose statement leads to a very speciiic ques- 



Pseudocentrotus depressus 



30 40 50 60 ^0 80 

 Time after fertilization 



120 130 



Fig. 3. Fluctuation of a TCA-soluble protein or polypeptide during the 

 division cycle in a sea urchin egg {Pseitdocoitrotus depressus), and non-fluctuation 

 of glutathione. Upper curve; total — SH soluble in 25 "o trichloroacetic acid. 

 Lines B, D, E, F, G. Soluble — SH after extraction of eggs with saturated 

 ammonium sulphate (B, D), after precipitation of protein from TCA extract (E), 

 and after dialysis of TC.\ extract. C shows oxidized glutathione (from Sakai and 

 Dan [35]). 



tion; is the intracellular environment ditTerent during division and 

 between divisions .' It was already demonstrated by Rapkine in 193 1 [32] 

 that the period of division was characterized by a remarkable fluctuation 

 in the soluble SH content of the cell; the "cycle" involved a striking 

 decrease in soluble SH during the early phases, up to about metaphase, 

 and an increase during the later phases. Rapkine identified the TCA- 

 soluble SH component as glutathione. The situation was confused when 

 attempts to confirm the glutathione cycle as such failed (e.g. [43]), but a 

 brilliant study by Sakai and Dan [35] resolved the problem. The cycle 



