58 Indications of a Central Zone of Development 



To one or other of these conclusions, either to the 

 presence of an undetected micronucleus or to the pos- 

 thumous action of the nucleus, one is necessarily driven, 

 as we said, by the fact that only the nucleated fragments 

 of an infusorian already completely developed, are capable 

 of regeneration. For this capacity of reorganization, 

 as one may call it, of the protoplasmic substance, 

 which gives to it again the form of the complete 

 individual but of correspondingly smaller size, cannot 

 possibly arise either from the properties of this sub- 

 stance itself or from its specific "physiological units" for 

 which the adult form of the individual would constitute 

 the only state of equilibrium. This is quite impossible 

 because a mass of protoplasm as large as the nucleated 

 part, but which contains no nucleus, does not manifest the 

 slightest tendency to regenerate, although it is capable of 

 surviving its ablation for several days. The impulse 

 tending to produce the specific form of the ordinary 

 equilibrium is present only when the nucleus is not 

 lacking. 



Nevertheless the material which disposes itself in this 

 definite specific form of equilibrium is not the nuclear ma- 

 terial but the protoplasmic. The nuclear substance with- 

 out participating itself in the process of reorganization, 

 merely provokes it in the protoplasmic substance, which in 

 this respect remains totally indifferent. This is dem- 

 onstrated among other things by the observation of 

 Gruber that the four nucleated fragments into which one 

 individual was cut by a transverse and a longitudinal in- 

 cision required in all cases the same time to regain their 

 complete specific form including the adoral ciliated zone. 

 The anterior end which already contained a portion of it 

 and which one would suppose to be better adapted in its 



