FERTILISATION 213 



and Simpson,! and more particularly by Calkins.^ The last 

 investigator found, further, that the periodic seasons of " de- 

 pression " or loss of vitality which invariably occurred if 

 conjugation were prevented, and which normally resulted in 

 the cessation of cell division and ultimately in death, could 

 be tided over and the race carried through further cycles of 

 activity by having recourse to artificial stimuli in the medium 

 surrounding the culture. In a series of experiments, which 

 Calkins conducted for twenty-three months with a single race of 

 Paramoscium, it was found that periodic reductions of vitality 

 occurred at intervals of about six months. At such times the 

 race under cultivation would have died out entirely had it not 

 been for the apphcation of stimuh in the form of extracts of 

 various food substances (beef, pancreas, brain, &c.). With the 

 assistance of these restoratives, on three separate occasions, this 

 particular race of Paramcecium was carried through four cycles 

 of activity and 742 generations without the occurrence of con- 

 jugation. It thus appears that a change in the environment 

 may result in a rejuvenescence of the race. 



As a consequence of these experiments, Calkins has sug- 

 gested that the purpose of conjugation may be to bring about 

 the union of individuals which have lived in different environ- 

 ments, and so to produce a renewal of vitality in the same kind 

 of way as a change in the environment itself. 



Calkins differs from Maupas in stating that diverse ancestry 

 is not essential in order that conjugation may occur, since he 

 obtained as large a percentage of successful endogamous as 

 exogamous pairings, and carried one endogamous ex-conjugant 

 through 379 generations. On the other hand, there is some 

 evidence that conjugation does not result in rejuvenescence when 

 both gametes have hved for a long time in the same medium, 

 so that their chemical composition is too similar.^ 



^ Simpson (J. Y.), "Observations on Binary Fission in the Life-History 

 of the Ciliata," Proc. Boy. Soc. Edin., vol. xxiii., 1901. 



^ Calkins, "Studies on the Life-History of Protozoa," IV., Jomj-. of Exp. 

 Zool., vol. i. , 1604. (Keferences to earlier papers are here given. See also Biol. 

 Bull, vol. xi., 1906.) 



' Cull, " Eejuvenescence as a Result of Conjugation," Jour, of Exp. Zool., 

 vol. iv., 1907'. Blackman ("The Nature of Fertilisation," British Assoc. 

 Reports, York Meeting, 1906) is of opinion that the rejuvenescence theory 

 of fertilisation is diflSoult to apply generally in view of the large number of 



