THE GERM-PLASM 8 1 



mid the via/uier of this distribution finds expression in the 

 architecture of each idic stage, or, as I have expressed it, in each 

 ' determinant-figure.'' 



It is obvious that the same geometrical figure may be con- 

 structed out of diiferent elements, just as the same form of 

 crystal may be produced from molecules of a different nature. 

 Thus the resemblance between the ontogenetic stages of nearly 

 allied species is to be explained by the degree of similarity 

 between their respective ' determinant figures,' which persists 

 althouijh the individual determinants constitutino; the figure 

 differ more or less from one another. As the study of develop- 

 ment shows, an explanation is thus offered of the fact that the 

 earlier ontogenetic stages are so very much alike in allied 

 species, and that the differences only appear later on ; for in 

 the early idic stages, the differences as regards the nature or 

 power of multiplication of single determinants, or groups of de- 

 terminants, can exert no marked influence, because the entire 

 number of determinants is still very large, and thus the archi- 

 tecture of the id will be practically the same in corresponding 

 stages. But the further ontogeny advances, and the smaller 

 the groups become into which the determinants separate, the 

 greater also will be the diversity in the architecture of the id, 

 and in the further distribution of ' unripe ' determinants resulting 

 from this architecture. Thus a certain part will be longer or 

 shorter, a spot of colour larger or smaller, and the final stages 

 of ontogeny — in which the cells possess only one determinant — 

 will differ according to the degree of difference which obtains 

 in the respective determinants. This explains the fact that the 

 segmentation cells in allied species are frequently exactly alike, 

 and also that the resemblance between many mammalian em- 

 bryos in their earlier stages, though not complete, is nevertheless 

 a very close one. 



The law of biogenesis, as far as it applies at all, depends on 

 the fact that phyletic development is partly due to new onto- 

 genetic stages being added at the end of ontogeny. In order 

 that these new stages mav be reached, the stages which were 

 previously the final ones must be passed through in each on- 

 togeny. This may be expressed in terms of the idioplasm as 

 follows: — the determinants of the id of germ-plasm become 

 endowed with a greater power of multiplication, so that each 

 one of them causes the addition of one or more cell-generations 



