THE MITOTIC CYCLE 



grasshopper embryo (Gaulden and Carlson^ ^^) while the correspond- 

 ing dose for the staminal hairs of Tradescantia is just above io~^ m 

 (Nebel and Ruttle^^s). It would be interesting to see whether a 

 nitronaphthalene which is effective on dividing plant cells at 2*8 X io~^ 

 M (Gavaudan and Poussel^^s) would inhibit the spindle in tissue cul- 

 tures at a greater dilution. 



The substance podophyllin, extracted from plants of the genus 

 Podophyllum is a mixture which contains two compounds, podophyllo- 

 toxin and quercetin, the former of which inhibits the spindle in the egg of 



\2 1 \1 2 3 V- S e 7 

 Before After anaphase begins 



S 10 11 12 13 



minutes 



Figure 68 Arrest of chromosome movement during anaphase 

 in cells of chick cultures by A 045 millimolar aminopterin, 

 and B 0*07 millimolar chloracetophenone. The continuous 

 curve represents the normal average movement of similar 

 cells in anaphase. In A the agent was added 2^ minutes 

 before anaphase, in 6 as soon as possible after anaphase 

 began; the movement was arrested within about thirty 

 seconds. In A neither cleavage nor nuclear reconstruction 

 ensued ; in 6 cleavage was irregular, and reconstruction was 

 partially inhibited. 



Arbacia at a dilution one hundred times greater than does colchicine 

 (CoRNMAN^***).* In tissue cultures, podophyllin has a marked inhibitory 

 effect on certain tumour cells at concentrations at which embryonic 

 cells are unaffected (Ormsbee, Cornman and Berger^*"^). 



These substances which act as spindle inhibitors at very low concen- 

 trations are all of molecular weights of the order of several hundred, but 

 exhibit no more general chemical features in common than do the 



* Cornman and Cornman^^''' have published a detailed study on the effects of these sub- 

 stances on cleavage in a number of marine eggs. 



