PERMEABILITY FACTOR IN DRUG ACTION 67 



Stances into Ascaris belongs to group three of the types 

 of permeation given on p. 52. The permeability of As- 

 caris cuticle to a drug can be readily calculated from the 

 equation P = o.zBjMy^ The figures obtained for the 

 permeability are sufficiently close to the experimental 

 ones to indicate the relative likelihood of various sub- 

 stances permeating in concentrations sufficient to have 

 a toxic action. For example, the results show that the 

 active substance hexyl resorcinol penetrates into Ascaris 

 much more rapidly than does the inactive substance re- 

 sorcinol. 



The Toxicity of Arsenoxides to Trypanosomes. The toxi- 

 city of arsenoxides to trypanosomes has been the subject 

 of an intensive study by a number of investigators, par- 

 ticularly King and Hawking. A great number of arsen- 

 oxides have been synthesised and their toxicities deter- 

 mined by finding the lethal dilution (L.D.), i.e. the 

 number of litres in which i gram mol. of each substance 

 must be dissolved to obtain a given degree of killing 

 of trypanosomes in a given time. The substances con- 

 cerned are of a type which we should expect to permeate 

 trypanosomes readily, and to conform to group four 

 of the permeation groups given on p. 52. The dilutions 

 in which these drugs are effective are very high, and it 

 seems likely from the studies of Hawking in particular 

 that practically every molecule of arsenoxide which 

 permeates into a trypanosome becomes fixed by a re- 

 Cell Physiology 5 



