138 RESPONSES OF CELLS ON THE BIOLOGICAL LEVEL 



be of interest would be those compounds which can 

 adhere more vigorously than does the virus to the regions 

 of the cell surface which are specifically virus-adsorbing; 

 then, if a substance like tryptophane is required for the 

 adhesion of a virus to a cell, a satisfactory inhibition of 

 virus activity might be obtained by using a competitive 

 substance such as methyl tryptophan. 



The relationship of mitotic poisons to mitotic stimu- 

 lators is also one which would probably repay more de- 

 tailed examination. Whilst it is possible that some stim- 

 ulating substances may be rather highly specific, other 

 substances which act as mitotic stimulators have a rel- 

 atively generalised effect. Xanthopterin has an effect 

 which is mainly restricted to the kidney and a smaller 

 effect upon the bone marrow. But oestrone has a rela- 

 tively generalised effect, producing an increase in mi- 

 totic rate in practically all cell lines capable of mitosis. 

 Despite this, oestrogens act as mitotic poisons for the 

 cells involved in cancer of the prostate and post meno- 

 pausal breast tumours. One wonders whether the mech- 

 anisms involved in mitotic poisoning and mitotic stim- 

 ulation are related or not. It is, of course, no new thing 

 to find that a substance may apparently have diametri- 

 cally opposite physiological effects under different cir- 

 cumstances: for example, adrenaline is vaso-constrictor 

 for some arterioles and vaso-dilator for other arterioles. 



