MAUPAS'S EESEAHCHES ON CTLIATE INPUSORTANS. 601 



Considering Weismaun's assertion^ that Protozoa are im- 

 mortal, it is of extreme interest to note that the later individuals 

 of a cycle differ from the preceding ones in a way that fully 

 justifies Maupas's use of the term " senescence." The later 

 individuals are reduced in size — sometimes to one fourth of 

 their normal length ; the buccal wreath is more and more 

 reduced as the stock ages ; the nuclear apparatus undergoes 

 degeneration in various ways^ rendering fertile conjugation 

 impossible. In this stage, however^ it is remarkable to find 

 that sometimes a sexual hypersesthesia is observed, inducing 

 conjugation between close relations^ though this is of course 

 sterile, and accelerates the death and disappearance of the 

 stock. This ultimate death by senescence took place in 

 Stylonychia pustulata in 316 generations from the excon- 

 jugate progenitor; while in a stock of Leucophrys patula, 

 of which the progenitor was possibly not an exconjugate, it 

 occurred only after 660 generations. 



In a section of the second paper these results are completed 

 by the proof that for many species, at least, the cycle shows 

 from its origin in conjugation to senescence first of all a stage 

 of '^ immaturity," in which conjugation cannot be induced; 

 then, after a certain number of bipartitions, "puberty'' is 

 reached, and during th^. period of "eugamy" that ensues fer- 

 tile conjugation can be induced at any time by suitable treat- 

 ment : this second stage merges into the last — of "senescence" 

 Puberty is reached in Stylonychia pustulata at the 130th 

 bipartition, in Onychodromus grandis at the 140th, and 

 in Leucophrys patula only at the 300th; and senescence 

 commences in the first about the 170th (though some senile 

 individuals showed as early as the 100th), in the second at the 

 230th, and in the third at the 450th. Since bipartition pro- 

 ceeds in some cases in senile individuals that have lost 

 their micronucleus, it is obvious that this organ plays no 

 necessary part in the vegetative reproduction of the Ciliata. 



