356 H. S. JENNINGS 



race; and we would find that, ultimately, evidences of weakened vitality 

 and degeneration appear in the aggregate of cells, and that they finally 

 die of old age (p. 103). 



It is conjugation that reinvigorates the stock; for succinct, 

 explicit statements of this we may quote from other papers of 

 Calkins and his associates : 



Conjugation between two cells results in the complete reinvigoration 

 of all activities, both physiological and germinal (Calkins and Cull '07, 

 page 376). As with the fertihzed egg of a metazoon, the copula or 

 fertilized egg of a protozoon is endowed with a great power of cell 

 reproduction and with a high potential of vitality, and this is the main 

 characteristic of the first period of the life cycle (Calkins '06, page 233). 

 As with the metazoon so with the aggregate of protozoa cells, we note 

 a period of youth characterized by active cell proliferation; this in both 

 groups of organisms is followed by the gradual loss of the division 

 energy accompanied by morphological changes in type of the cells pre- 

 liminary to conjugation and fertilization and to the renewal of vitality 

 by this means (Calkins '06, p. 232). 



The experiments described in the present paper constitute an 

 examination as to how far conjugation actually exhibits these 

 effects in Paramecium; as well as how far it shows other results. 

 We shall here summarize and discuss the evidence as to the 

 effects of conjugation on the rate of reproduction; on the vigor 

 or vitality, as evidenced by the comparative mortality; on 

 abnormalities; on its production of variation; on inheritance; 

 and the relation of the results as a whole to the theory of reju- 

 venescence. 



EFFECT OF CONJUGATION ON RATE OF REPRODUCTION 



Practically all the experiments show that the average rate of 

 reproduction is less after conjugation than before. That is, if 

 we take two sets of animals of the same stock and history, both 

 ready to conjugate; permit one set to conjugate, and prevent the 

 other, we find that those which have conjugated divide there- 

 after on the average less rapidly than the others. 



In most cases the rate of fission was very considerably greater 

 in those that had not conjugated, the excess usually varying from 

 25 per cent up to 80 per cent or more. In some cases, however. 



