No. 593] 



THE PROTOZOAN LIFE CYCLE 



263 



cirri which are regenerated are the visible expression of 

 inherited structures characteristic of the species. Since 

 the enucleated fragment from a cell cut when young does 

 not regenerate while the nucleated fragment does, we 

 must conclude that one essential factor at least, necessary 

 for the production of these inherited structures, lies in the 

 micronucleus. 



The giant cirri, furthermore, are visible differentiations 

 which are precociously formed at division. This must 

 mean that the inherited factors find their expression at 

 this period, and it follows from the successful formation 

 of giant cirri in enucleate fragments from old colls, that 

 whatever may be the direct causative agent or agents in 

 the process they must be generally distributed throughout 

 the protoplasm at this time. We have no direct evidence 

 as to what these agents may be ; possibly there is only one 

 and that of the nature of a specific enzyme, or perhaps 

 some chemical body analogous to hormones formed as a 

 result of mutual interaction of nucleus and cytoplasm 

 when the latter has reached a certain stage of chemical 

 differentiation through normal activities. Or it is possible 

 that such chemical bodies are present at all times and are 

 activated only when the protoplasmic substratum reaches 

 some particular stage of development. Thus it is possible 

 that, with continued metabolism, the acidity of the proto- 

 plasm gradually increases until a concentration is reached 

 in which specific enzymes, not able to act before, are now 

 activated. 



However theoretical the interpretation of the phenom- 

 enon may be, the periodic and temporary power of re- 

 generation is an observed fact indicating a difference in 

 the protoplasmic make-up at different age periods, a dif- 

 ference which may be satisfactorily expressed by the 

 phrase cumulative chemical differentiation. 



Another observed fact is that the regenerative power 

 is exhausted with cell division, for young enucleated frag- 

 ments do not regenerate. This indicates a reduction of 

 the differentiated adult protoplasm to the condition of 

 young cells ; or, at least, the protoplasm is restored to a 



