CHAPTER THREE 
PHENOMENA WHICH POINT TO THE EXISTENCE OF A 
CENTRAL ZONE OF DEVELOPMENT—HYPOTHESIS OF 
THE STRUCTURE OF THE GERMINAL SUBSTANCE, 
i. Phenomena Which Point to the Existence of a 
Central Zone of Development 
The only group of organisms in which one can say 
that the existence of a central zone of development is di- 
rectly demonstrated is that of the unicellular organisms, 
in which this zone is constituted by the nucleus. In 
pluricellular organisms it is only indirectly that we are 
able to arrive at the conclusion that the central zone 
exists. 
Experiment has shown that the necessary and suff- 
cient condition for the ontogenetic development of the 
Infusoria is the presence of a nucleus. This latter con- 
stitutes therefore for them an effective central zone of 
development and consequently ontogenesis consists in 
them in a true and proper centroepigenesis. 
If one divides an amoeba or a rhizopod or an infuso- 
rian already completely developed, into many pieces, that 
one of these fragments which remains provided with its 
nucleus though it be the smallest of all, is yet capable of 
reproducing by new formation all the missing organs and 
of developing again into a normal individual; whereas the 
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