No. .333] NUCLEUS AND CYTOPLASM IN HEREDITY 299 



to be due to the fact that the chemical systems i?i such a 

 giant molecule are never fully saturated at any one time, 

 so that there is continually an adding and detaching and 

 shifting of side-chains with perhaps at times more funda- 

 mental shifts or replacements in the amino-acid "nuclei" 

 themselves. Quantitative and qualitative differences of 

 proteins would seem to depend fundamentally on the kind 

 and amount of the constituent amino-acids and second- 

 arily on the chemical nature of the various side-chains. 



Probably the scheme as outlined is much simpler than 

 the true conditions in the protein molecule, but it will 

 serve as a sort of diagram of the relations which exist 

 there. It is probable, too, that the conditions in differ- 

 ent proteins vary greatly in complexity. The chief point 

 to be emphasized is the fact that the results of many in- 

 vestigators bear out this general conception of the pro- 

 tein molecule. 



It would seem then that in the light of our knowledge 

 of the complex molecular configuration of the proteins, 

 the substances which appear to be the most intimately 

 concerned with life phenomena, we have, without resort- 

 ing to the idea of mysterious separate entities, ample 

 basis for that peculiar handing on of metabolic energies 

 already established which we term heredity. The mech- 

 anism of heredity would seem to be not so much a local 

 problem of nucleus or cytoplasm as of (1) fundamental 

 species substances, probably mainly protein in nature, 

 together with (2) equally specific enzymic substances 

 which regulate the sequences of the various chemical and 

 physical processes incident to development. As develop- 

 ment progresses, more and more kinds of chemical prod- 

 ucts are released and in consequence an increasing num- 

 ber of chemical reactions are set going. After the 

 germ becomes multicellular such new factors must be 

 reckoned with as the influences, mechanical, chemical, etc., 

 of the various parts of the body on one another. And 

 < ' v, 'ii with our present meager knowledge of hormones 

 we can see that this may be no inconsiderable factor in 

 modifying the developing organs in complex organisms. 



