86 Hypothesis of Structure of Germ Substance 



common in plants, of the retention by some cells of the 

 germinal capacity even though they belong to somatic 

 tissues which have already advanced to a certain degree 

 of differentiation. 



In the same way is easily explained how a given 

 piece of a hydra or medusa reorganizes itself so as to 

 reproduce the entire individual without any correspond- 

 ing increase of its mass. 



For since histologic differentiation in the hydra is 

 not very pronounced one can surmise a priori that in 

 all or nearly all their cells, the whole of the specific 

 potential elements must coexist with the somatic elements 

 peculiar for each cell and acquired by it during develop- 

 ment. The separation of the fragment from all the rest 

 of the organism, which arrests the general circulation 

 of nervous energy, will therefore cause the somatic 

 elements which were active in the intact individual to 

 return to the exclusively potential state and thus enable 

 the germinal elements to become active again. That 

 cell or group of cells which surpasses the others in vigor 

 will have its germinal elements activated first and will 

 then form a central zone directing development of the 

 others; and the distribution of nervous energy, which 

 again passes through the wonted series of ontogenetic 

 stages, will now proceed in the fragment in the same 

 way as formerly in the entire individual. 



There are often external circumstances which deter- 

 mine what cells of the fragment shall constitute the central 

 zone. Thus if one cuts off from the trunk of the hydra 

 both the tail end and the head end at the same time, 

 and then places the fragment with the lower cut surface 

 down, the head is reproduced at the same end as formerly, 

 whereas if one turns it over so that the former head end 



