368 H. S. JENNINGS 



sometimes been held to constitute a visible rejuvenescence, a 

 rejuvenescence of the macronucleus. This in some forms is 

 accompanied by a renewal of other structures, as for example, 

 of the bodily appendages in the Hypotricha. Engelmann ('76) 

 emphasized these changes as constituting in themselves ,''ein 

 wahren Verjiingung" (p. 629). 



This actual replacement of old structures by new no one will of 

 course deny, and it seems not inappropriate to call it a reju- 

 venescence, if we mean by this word nothing more than these 

 observed facts. 



2. But the theory has as a rule gone far beyond these observed 

 facts. Thus, Maupas says, after a statement of these structural 

 changes : 



Ce nouvelle appareil nucleaire agit siir tout rorganisme, auquel il 

 appartient, comme une sorte de ferment regenerateur, lui restituant, 

 sous leur forme parfaite et integrale, toutes les energies vitales carac- 

 teristiques de Tespece. Get etre se trouve done rajeuni dans le sens 

 litteral et absolu du mot. II pent des lors redevenir le progeniteur 

 d'un nouveau cycle de .multiplications agames, dont toutes les gene- 

 rations successives seront douees des memes facultds rajeunies, jusqu'a 

 ce que celles-ci s'usent et s'affaiblissent peu a peu, par leur exercice 

 meme, et en arrivant ainsi k ressentir le besoin reparateur d'une nouvelle 

 periode d'activite fecondatrice ('89, p. 434). 



Now, in the passage, we have quoted, Maupas evidently 

 affirms certain things that by no means follow from the structural 

 changes observed, but can only be demonstrated by the results 

 of experimentation. We shall have to inquire how far these have 

 been thus demonstrated. But before doing this, we must pro- 

 ceed to one farther development of the theory. 



3. As we have seen (page 357), Maupas did not hold that the 

 vigor and rate of reproduction are increased by conjugation, 

 although such a general statement as the one above quoted would 

 seem to imply that this is true. This idea has, however, been 

 held by many as a fundamental part of the theory of rejuve- 

 nescence. The rate of reproduction has been held to become 

 less and less as the number of vegetative generations increases, 

 until by a new conjugation it is brought back again to its original 

 level (see the quotations on page 356). 



