OPT. ACT IT., HEREDITY AND ENVIRONMENT 75 



thermore, in the fundamental protoplasmic systems, there 

 are devices to avoid racemization, which were discussed 

 previously; similarly it seems that there are devices to 

 avoid inversion, such as those regulated by the ''prin- 

 ciple of fixed pathway". Neither of these devices oper- 

 ates in the transformations of secondary protoplasmic 

 constituents. 



SUMMARY 



1. It is not possible to invert, by external influences, 

 the asymmetric structure of the primary constituents of 

 protoplasm which is the result of a long evolutionary 

 process. 



2. As to secondary substances of protoplasm such as 

 products of metabolism (for example, lactic acid in fer- 

 mentative processes), the sign of their optical activity 

 also represents a fixed hereditary character of the spe- 

 cies or strain which elaborated them, but external in- 

 fluences can affect the catalytic racemization of these 

 products which were initially formed in the optically pure 

 state. 



3. Concerning the mechanism by which the produc- 

 tion of a given optical isomer is controlled in metabolic 

 activities, one should note that: (a) From the same 

 symmetric initial substrate, enzymes of ditferent organ- 

 isms can synthesize optically dilferent substances; (b) It 

 is often through optically inverse catalysts that optical 

 antipods are synthesized; (c) Some catalysts can, after 

 chemical alterations which do not constitute an inver- 

 sion, synthesize substances in a form which is the optical 

 inverse of the form that was synthesized before the al- 

 teration of the catalyst; ( d) The sign of the optical rota- 

 tion of the final product of a series of metabolic reactions 

 may depend on the "pathway" followed in intermediate 

 reactions; (e) Inversions of the Walden type (transfor- 

 mation of one optical isomer into its antipod in a series 



