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 
