No. 579] 



PROGRESSIVE EVOLUTION 



L69 



so as to bring about corresponding changes in the adults 

 into which the eggs develop. 



There seems therefore to be no great difficulty in com- 

 prehending, at any rate in a general way, how the egg 

 may become the repository of definite chemical sub- 

 stances, organ-forming substances if we like to call them 

 so, possibly to be classed with the hormones and en- 

 zymes, which will influence the development in a particu- 

 lar manner as soon as the appropriate conditions arise. 



Unfortunately, time will not allow of our following up 

 this line of thought on the present occasion, but we may 

 notice, before passing on, that with the accumulation of 

 organ-forming substances in the egg we have introduced 

 the possibility of changes in bodily structure, to what- 

 ever cause they may be due, being represented by cor- 

 related modifications in the germ-cells, and this is doubt- 

 less one of the reasons why the germ-cells of different 

 animals are not all alike with regard to their potentiali- 

 ties of development. 6 



We now come to the question of how the nucleus of the 

 germ-cell acquired its great complexity of structure. 

 We are not concerned here with the origin of the dif- 

 ferentiation into nucleus and cytoplasm and the respec- 

 tive parts played by the two in the life of the cell. The 

 problem which we have to consider is the complication 

 introduced by the sexual process, by the periodically re- 

 curring union of the germ-cells in pairs, or, as Weis- 

 mann has termed it, amphimixis. This is well known 

 to be essentially a nuclear phenomenon, in which the so- 

 called chromatin substance is especially concerned, and 

 it is a phenomenon which must have made its appear- 

 ance at a very early stage of evolution, for it is exhibited 

 in essentially the same manner alike in the higher plants 

 and animals and in unicellular organisms. 



Let us suppose, for the sake of argument, that when 

 amphimixis first took place the chromatin of each germ- 



« Compare Cunningham's " Hormone Theory of Heredity " (Archiv fur 

 Entwicl'lungsmechanilc der Organismen, Bd. XXVI, Heft 3). 



