1 6 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 



