1899] REGENERATION 321 



has been recently developed by Oscar Hertwig. 1 I still believe as 

 firmly as ever that the germ-plasm is made up of determinants, — that 

 is, of groups of vital particles, the presence of which in the germ-plasm 

 conditions the formation of definite parts of the organism, and the 

 variation of which modifies these parts alone. The conception of the 

 germ-plasm as composed of ids is also in no way affected by the newer 

 facts, and it still seems to me not only a fruitful but an imperative 

 hypothesis. 



As far as the theory of heredity is concerned, it is a matter of 

 indifference whether the determinants of the germ -plasm remain 

 together during ontogeny, and are only individually excited to activity 

 by specific stimuli, or whether they are dispersed during ontogeny 

 into smaller and smaller groups. Even in the latter case liberating 

 stimuli are also indispensable, and it does not affect the real problems 

 of heredity — the inheritance of functional modification, the mingling 

 of parental characters, reversion to nearer or more remote ancestors, and 

 so on — in what particular way the individual determinants are excited 

 to activity, provided that their existence is established. That they do 

 exist, I consider certain, because it follows as a logical necessity from 

 the facts of inheritance and variation. Determinants must be present 

 because the parts of the organism are capable of individual and trans- 

 missible variation, and this is only possible if there are in the germ- 

 plasm living particles related to definite parts of the perfect organism ; 

 they must be present, not in the sense of the " Anlagen " of the old 

 " evolution " or preformation theory, in which they were regarded as 

 being themselves the part in question although only in nuce, but in the 

 sense of working, living particles which operate during the course of 

 development in a definite manner, so that the- part they have to determine 

 is thereby produced. 



It is only the theory of ontogeny which is affected by the fresh 

 facts, not that of heredity. The experimental determination of the 

 prospective importance of the blastomeres which we owe to Koux, 

 Driesch, Wilson, Chabry, and so many others, and the now indubitable 

 process of post -generation as formulated by Eoux, necessitate the 

 assumption that the blastomeres commonly contain all the determinants 

 within themselves, and that the cells of the external and middle germinal 

 layers retain many of them for a long time. But it by no means 

 follows that a pure liberation-theory (Auslosungs-Theorie), like that 

 put forward by Oscar Hertwig, is correct. It is much more probable 

 that at some period of development, at least in the case of higher 

 animals, a limitation in number of the " Anlagen " takes place in the 

 cells during ontogeny, and this may often go so far that they eventually 

 only contain the one " Anlage " which determines their differentiation. 

 I cannot agree with the statement in Hertwig's latest book, in which the 

 supposition is made that all the cells in ontogeny contain the complete 



1 "Die Zelle und die Gewebe " (II.), Jena, 1898. 



22 NAT. SC. VOL. XIV. NO. 46. 



