THE MITOTIC CYCLE 



and Kriszat^^^ find that adenosine triphosphate increases cytoplasmic 

 rigidity in the egg of Strongjilocentrotus, and improves the percentage of 

 cleavage in batches of fertilized eggs. Probably in general, purines and 

 their nucleosides and nucleotides may exert either inhibitory or 

 stimulatory (Hopkins and Simon-Reuss^^^) effects on growing tissues, 

 depending upon the concentrations applied. 



Inhibition of mitosis during prophase 



It is rare that cells in mitosis under the influence of chemical agents 

 are held up during prophase. One instance of this is apparently provided 

 by the sea-urchin egg. Clowes and Krahl^^^ found that fertilized eggs 

 treated with 4, 6 dinitro-o-cresol are arrested at a stage in which the 

 chromosomes can be recognized, the nuclear membrane is still intact, 

 and the asters have just been formed; Runnstrom^^^ 226 found that 

 hydrocyanic acid has a similar effect. Respiration is stimulated by the 

 former agent and depressed by the latter, yet the effect of both on 

 mitosis is similar. Scott^^' finds that cyanide inhibits the swelling of 

 the male pronucleus after its fusion with that of the egg. 



In a recent review, Krahl^^^ concludes that the tendency of sub- 

 stituted phenols to inhibit sea-urchin eggs in prophase relates merely to 

 the fact that this phase of mitosis is the longest; for the chance of 

 arresting a cell in a particular stage must be proportional to its normal 

 duration where there is no period of special sensitivity during the whole 

 cycle. Blumenthal^^^ concluded that the action of cyanides on the 

 fertilized Arbacia egg shows no special phase specificity. Prophase in the 

 eggs of Arbdcia is readily prolonged by irradiation given prior to this 

 stage, during the period of fusion of the male and female pronuclei 

 (Henshavs^;^^^^ Henshaw and Cohen^^^^). Pre-prophase is also a period 

 of special sensitivity of the eggs of Asterias to nitrogen mustards (Can- 

 nan, quoted by Friedenwald^^s'^). 



Ephrussi et alii^^'^ claim that at low tensions of oxygen, cells in tissue 

 culture are arrested in prophase. None of the chemical agents applied 

 to tissue cultures by Hughes^^^ ^"^ ^^^ have had this effect. Erickson 

 and Rosen^^'^ find that after treatment of the seedlings of ^ea mais 

 with the unsaturated lactone protoanemonin, the meristematic nuclei 

 are nearly all either in interphase or early prophase, though a few are 

 apparently in late prophase. No later stages of mitosis are to be seen. 

 According to Loveless and Revell^^^ this substance has 'cytological 

 effects similar to the mustards'. D'Amato^^^ refers to other examples 

 of the arrest of plant cells in prophase. However, from the available 

 data, it appears that prophase is the longest phase of mitosis in plant 

 cells (Table HI, p 85), and it is possible that arrest at this stage may 

 be related to its duration, rather than to special sensitivity. 



190 



