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SCIENCE PROGRESS 



In fig. la the side-chains are represented by the unshaded parts 

 a, b, and c, while the molecules of nutritious substance are repre- 

 sented by d, f, and g. It is obvious that the presence of the 

 molecule / on b will not interfere to any extent with the action 

 of a or c. This symbolises the normal action of the side-chains. 

 If, however, the central side-chain b should happen to anchor 

 the abnormally large body/ 1 (fig. i b), the state of affairs would 

 be quite different ; for f 1 occupies so much room that the side- 

 chains a and c could not come into play, and the cell would 

 consequently be starved. On Schryver's view, then, toxicity in 

 a tissue may be regarded as due to a steric hindering of normal 

 functions. 



Before concluding this article, some mention should be 

 made of the relation between vital action and the production 

 of molecular asymmetry. When in the laboratory we con- 

 vert a compound Cabc 2 into another, Cabcd, which contains an 

 asymmetric carbon atom, it might be expected that an optically 

 active product would result. In practice, however, this does 

 not occur; for instead of one antipode being produced, the final 

 product is racemic. A consideration of what takes place in the 

 course of the change from Cabc 2 to Cabcd will make clear the 

 reason for this. In the substance Cabc 2 , we are dealing with a 

 tetrahedral arrangement of atoms like that shown below : 



