C. DOBBLL 141 



caused by transferring animals from a culture containing plenty of food 

 into one containing none or little — induces conjugation, provided that 

 the animals are " karyogamically mature," and unrelated. In cultures 

 containing closely related animals it brings about encystation or abortive 

 attempts to conjugate. If the organisms are too richly nourished during 

 the eugamic period, conjugation does not occur. 



21. Senescence, which occurs in the last period of the life-cycle in 

 organisms which have failed to conjugate, is manifested by the following 

 phenomena : decrease in the size of the organisms, degeneration of the 

 nuclear apparatus (in various ways), reduction of the mouth-parts and 

 appendages, physiological degeneration (loss or inhibition of various 

 functions), and sometimes by " morbid sexual hyperaesthesia," leading 

 to abnormal and sterile conjugations between closely related individuals. 

 The final result is always death. 



22. Maupas did not share the view — which had then recently been 

 revived by Weismann' — that the Ciliata are "immortal." He believed 

 that his researches had shown that these animals are not capable of 

 multiplying by fission for an endless period. Unless conjugation takes 

 place at the proper season, the doom of the organism or its offspring 

 is sealed, and old age and death are as inevitable for Parcmiecimn as 

 for man. 



CHAPTER III. 



The Results of Later Investigators. 



23. Having briefly outlined the conclusions reached by Maupas, 

 I shall now review the chief results which have accrued from the 

 investigations of subsequent workers. For the sake of convenience, 

 I shall sub-divide these results into two main groups — according as 

 they concern (A) the asexual or (B) the sexual period in the life-history: 

 and I shall consider the experimental results, in each subject studied, 

 as far as possible in chronological order. 



' Weismann's conception of the "immortality" of the Protozoa was familiar to earlier 

 investigators — e.g., Ehrenberg and Diijardin. The first expression of this idea with which 

 I am acquainted is in the work of the former (Ehrenberg, 1838). Moreover, he clearly 

 realized the allegorical nature of this "immortality" — "welche poetisch genug an Unsterb- 

 lichkeit und ewige Jugend grenzt. " "Man theile sich in zahllose neue Theile, um 

 zahllose Jahi-e zu leben und jung zu seyn." Woodruff (1909 a), however, is wrong in 

 stating that Ehrenberg held that the "Protozoa are so simply organized that they are 

 not subject to natural death." Calkins (1902a) is also wrong in stating tliat "Ehrenberg's 

 view was vigorously contested" by Dujardin. (Cf. also footnote to § 17, supra.) 



Journ. of Gen. iv 10 



