Certain Regenerations ill 



viving half preserves as is shown by Roux's figures, the 

 position which it would have had in normal development. 

 Then after segmentation has taken place, the same form- 

 ative factors act on each particle and on each blastomere 

 respectively, which would have acted upon them in nor- 

 mal development, consequently also the same forms result ; 

 Ergo : half-embryo." 73 



From the epigenetic standpoint this explanation is 

 inadmissible. For when the half embryo begins to take 

 on the characteristic form of its species, and thereby 

 indicates as we have seen that the specific action of the 

 germ substance has from that time become preponderant 

 over that of the deutoplasm, one could not assert that 

 the same organ forming factors continue their action, for 

 that would be to deny that there is any formative action 

 at all exerted by the idioplasmic nuclear substance of one 

 entire half, left or right, anterior or posterior, upon the 

 other, developing half. This would be exactly the op- 

 posite of what simple epigenesis postulates, for it 

 attributes the tendency of development to take on its 

 specific form of equilibrium to the reciprocal action of 

 all the innumerable little masses of one and the same 

 idioplasm, which are active at the same time in all the 

 nuclei of the entire organism. 



Roux then can rightfully assert that the half embryos 

 constitute by themselves the most direct and decisive 

 refutation of the theory of epigenesis. 



If we pass on now from half embryos to the regen- 

 eration of amputated organs, we know that this con- 

 stitutes one of the most important arguments that the 

 epigenesists ordinarily advance against pre formation. 



78 Driesch : Analytische Theorie der organischen Entwicklung. 

 Leipiz, Engelmann, 1894. P. 15 16. 



