16 Biogenetic Law of Recapitulation 
morphological and physiological state of the other part of 
the organism at that moment; for in the corresponding 
phylogenetic state the two portions were in perfect 
equilibrium with each other. It is necessary then to sup- 
pose that somewhere in the remaining parts of the or- 
ganism, there enters into play just at that moment and 
only at that moment, some factor which was not present 
in the ancestral species. 
Further, since the alteration in the organism during 
ontogeny is not confined to a single part of the organism 
but affects several parts at the same time, and since the 
impulse which comes into play at the end of each stage of 
development compelling the passage to the successive 
stage must lie external to each of the parts undergoing 
transformation, it cannot lie in any of these parts. 
This will be possible, however, only on condition that 
among all the different parts of the organism there is at 
least one part which is not itself subject to any substantial 
change, but in which there comes into activity a series of 
specific energies one after another of which each provokes 
the passage of all the other parts of the organism to the 
next ontogenetic stage. 
This special part may be called the central zone of 
development. And one can give the name of centro- 
epigenesis to this hypothesis by which ontogenetic 
development is made to depend on an infinite number of 
different influences which this zone gradually exerts upon 
all the remainder of the organism by activating succes- 
sively a regular series of specific energies, each remain- 
ing in a potential state up to the time of its activation. 
Now the part which actually does remain unaltered 
from the first segmentation of the egg up to the giving off 
of the reproductive cells by the new organism is the 
