564 SCIENCE PROGRESS 



The Chemical Basis of Development 



Development, the arrest of which leads indirectly to the 

 formation of tumours, like growth, is the outcome of the chemical 

 changes that take place in living tissues — the changes upon 

 which their working and their life depend. This is acknow- 

 ledged in the case of growth. Foreign substances come into 

 contact with a living cell. They are absorbed by it and 

 broken up in it by catalytic agents. The products of this 

 catalysis are built up again in a different combination, and if 

 the addition exceeds the consumption the result is, first, 

 increase in size, and then, if circumstances are favourable, 

 increase in number, each newly formed cell possessing the 

 same power as the parent of which it once was part. 



The same thing is true of development. Development, 

 like growth, is the outcome of chemical changes that take 

 place in the living cell, but there is this difference, that while 

 growth depends upon the maintenance of all the reactions in 

 their relative proportions, development depends upon the 

 predominance of one. Development is the result of one of 

 these chemical reactions, or rather of one series of them, 

 being carried on by one part of the body in excess of all the 

 rest for generation after generation. Division of labour means 

 that every part of the body has to undertake a special kind 

 of work. Special work entails a special chemical reaction or 

 series of reactions. The more thoroughly the work is done, 

 the more completely does this special reaction predominate 

 over all the others at that particular spot. This, continued 

 in the same group of cells for generation after generation, of 

 necessity involves progressive modification of chemical con- 

 stitution and of structure, or in other words development. 



The layer of cells, for example, on the exterior of the 

 organism, always exposed to foreign influences, becomes 

 modified in course of time. They become harder and more 

 resistant, keratin or some similar substance forms in them, 

 and as generation succeeds generation and a similar modification 

 takes place in each, at last an outer protecting layer is developed 

 characteristic of that particular kind of organism. The same 

 thing is true of the cells that line the digestive tract. Their 

 special work, or one part of it, is the manufacture and dis- 

 charge of catalytic ferments. The chemical reactions upon 



