So THE GERM-PLASM 



The species of a genus, and often the genera of a family, cannot 

 be distino^uished from one another bv the number of cells com- 

 posing them, nor by an increase in the variety of these cells, 

 but only by qualitative differences in the structure of the various 

 parts. Hence the phyletic development of living beings cannot 

 simply be due to the augmentation of the number of determinants 

 in the id of germ-plasm, but must also be attributed to a change 

 in the nature of the determinants and in that of their com- 

 ponent biophors. 



The structure of the idioplasm which we have here assumed, 

 also offers an explanation of the phenomena of parallelism 

 between oitogeny and phytogeny, which depend on the law of 

 biogenesis as well as on the relegation of thejinal characters to 

 earlier and earlier ontogenetic stages in the course of phylogeny. 

 Let us first consider the former of these phenomena. We have 

 assumed that each ontogenetic stage is characterised by a definite 

 ' determinant figure,' i.e., a sort of geometrical structure com- 

 posed of the determinants. The nature of each individual cell 

 is certainly controlled by those determinants in the nucleus 

 which have reached maturity, — that is to say, have arrived at 

 the stage in which they break up into biophors and migrate into 

 the cell-body. But the manner in which the embryonic develop- 

 ment of an animal occurs does not by any means depend only 

 on the histological structure of the individual cells in each stage, 

 — it rests to a much greater degree on the manner in which 

 these cells divide and on the rate of their division, and also 

 primarily on the way in which the ' unripe ' determinants of the 

 nuclear substance, which are still latent, are grouped together 

 and distributed by means of the cell-divisions. This distri- 

 bution of the primary constituents among the different cells is 

 of the first importance in determining the character of the 

 ontogeny ; and one could easily imagine a case of animal 

 embryogeny in which ten or twenty generations of similarly 

 constituted ' embryonic cells ' followed each other, and in which 

 a perfectly definite distribution of the primary constituents 

 (determinants) had nevertheless occurred, although only now 

 apparent for the first time. It is well known how close a 

 resemblance exists between the cells of the embryo in various 

 stages in the case of the higher animals. 



The regular distribution of the determinants which are still 

 latent or ' unripe ' must therefore decide the course of ontogeny ; 



