Certain Regenerations rit 
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.” 78 
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 ali 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 preformation. 
Driesch: Analytische Theorie der organischen Entwicklung. 
Leipiz, Engelmann, 1894. P. 15—16. 
