212 ESSENTIALS OF BIOLOGY 



i. It is not a material substance. 



Weismannian determinants and, according to the cruder views, 

 the genes of Morgan have been regarded as elementary constituents 

 of the chromatin of the nucleus of the ovum about to develop. 

 This substance, a nucleo-protein is supposed to undergo chemical 

 and physical changes, of itself and in virtue of its chemical and 

 physical nature and quite apart from any other agejicy so that the 

 embryo comes into existence. The nucleo-protein must, there- 

 fore, (a) be able to select materials from the environment and 

 chemically transform these into materials similar to itself ; (b) 

 grow and reproduce by dividing itself and then the cell containing 

 it, growing again and then again dividing and so on ; (c) orientate 

 the angles of the division-planes, and divide more rapidly at some 

 places than at others so that an assemblage of cells of definite, 

 specific form comes into existence. 



The physical analogy with these processes is crystal-growth. 

 But the crystal-groups, such as the ice-flowers on a window-pane, 

 " crystal-trees," etc., are random assemblages of crystals, all of 

 w hich are of the same chemical individuality and geometrical form, 

 whereas the assemblage of cells that is the fully-developed product 

 of the hypothetical developmental material contains substances 

 that are chemically different, and are chemically more complex 

 than nucleo-protein. Further, this assemblage has a perfectly 

 specific form. Again the constituents of the ovum, embryo and 

 developed organism are colloids and not crystalloids. It will be 

 clear, then, that candid examination of the processes of crystal- 

 growth does not bear out the analogy. There are no chemical 

 or physical processes that suggest how a physical- chemical system, 

 of itself can so react with its environment so that the processes, 

 (fl), (b), and (c) above, occur. 



It is very curious that the view that the developmental agencies 

 are material substances should have been held in spite of the 

 elaboration of modern thermodynamical theory. Consider a 

 physical-chemical system (such as nucleo-protein). If it under- 

 goes chemical change (decomposes, hydrolyzes, oxidizes, etc.) of 

 itself it does so only when energy is dissipated in so doing. This 

 means that such " spontaneous " changes lead to energy- dissipa- 

 tion, to the decomposition of the substance into other substances 

 which are chemically simpler, to chemical, statical equilibrium 

 and to entropy-increase. After such reactions have occurred the 



