ASYMMETRIC ANALYSIS 127 



meclianism of toxic action, (2) the evolution of the ner- 

 vous system, (3) the mechanism of various physiological 

 functions in protozoa, 



2. The two optical isomers of a toxic substance may 

 exhibit different degrees of toxicity (the natural isomer 

 being- more toxic) but possess the same mechanism of 

 toxic action, as judged by the identity of the relation of 

 increasing toxicity to concentration and by the identity 

 of the temperature characteristics. Such conditions have 

 been observed, in particular, in nicotine. There are cases 

 in which none of the two relations just mentioned hold. 

 The last series of cases cannot be accounted for by the 

 assumption of a receptive substance diversely affected 

 by the two isomers. 



3. The coefficient of relative toxicity of the two isomers 

 of tartaric acid increases from 1 to 1.305 when one 

 passes from the protozoa to the fishes through the worms 

 and the Crustacea. The killing action, in the lower forms, 

 seems, then, to be due to factors which are common to the 

 two isomers, while, in the higher forms, it is due to factors 

 which differ in the two isomers. It is suggested that the 

 factors of the first type are those which act mostly on the 

 surface of organisms, and the factors of the second type, 

 those which act internally. The problem of the mode of 

 action of toxic substances is then linked to that of the 

 evolution of the integuments in fresh water animals. 



4. The study of the toxic action of nicotine in animals 

 of variously developed nervous systems points to the 

 absence of a spatially specific receptive substance in Pro- 

 tozoa, Coelenterata, Turbellaria, Rotatoria and Nemer- 

 tinea, and to the presence of such a substance in Annelids, 

 Chaetognatha and Vertebrates. In Arthropoda it is ab- 

 sent again. A comparison of its distribution with that of 

 acetylcholine in different groups of animals leads to sig- 

 nificant data on the evolution of the nervous system. The 

 receptive substance in nicotine poisoning shows some 

 close relation to the receptive substance for chemical 

 mediation in the transmission of the nerve impulse. 



