REPRODUCTION 169 



As the generations are multiplied, the general decline in vitality 

 is manifest not only in the decreased division-rate, slow growth, 

 abnormal form and function of certain organellae, etc., but also in 

 inability to complete the process involved in conjugation. Jennings 

 found that when individuals of aging stock of Paramecium hursaria 

 conjugate with those of a young vigorous stock, certain numbers of 

 exconjugants die without multiplication, or give rise to weak and 

 abnormal descendants. As the stock grows older, the number of ex- 

 conjugants that die or are weak increases until a period arrives at 

 which time all the exconjugants die without multiplication. How- 

 ever, if conjugation takes place before the decline in vitality becomes 

 too great, such stocks in some cases are able to restore to high vital 

 ity. Endomixis and autogamy have also been considered to have a 

 similar effect on the vitality of the progeny of the individuals in 

 which they occur. Experimental data indicate that conjugation of 

 still vigorous young stocks does not bring about any greater vitality 

 to their progeny, but that of older stocks results in many strains with 

 restored vitality. Sonneborn (1942) mentions the appearance of 

 varied characters after macronuclear regeneration (p. 129) in P. 

 aurdia. The animals become smaller and of different form; they 

 reproduce more slowly ; they are less viable and die out after a period 

 of few weeks or months. Thus regenerated macronuclei are unable 

 to produce high vitality. 



It is probable that the process of replacing old macronuclei by 

 micronuclear material which are derived from the products of fusion 

 of two micronuclei of either the same (autogamy) or two different 

 animals (conjugation), would perhaps result in a complete elimina- 

 tion of waste substances from the newly formed macronuclei, and 

 divisions which follow this fusion may result in shifting the waste 

 substances unequally among different daughter individuals. Thus in 

 some individuals there may be a complete elimination of waste 

 material and consequently a restored high vitality, while in others 

 the influence of waste substances present in the cytoplasm may offset 

 or handicap the activity of new macronuclei, giving rise to stocks of 

 low vitality which will perish sooner or later. In addition in conjuga- 

 tion, the union of two haploid micronuclei produces diverse genetic 

 constitutions which would be manifest in progeny in manifold 

 ways. Experimental evidences indicate clearly such is actually the 

 case. 



In many ciliates, the elimination of waste substances at the time 

 of binary fission and sexual reproduction (conjugation, and autog- 

 amy), seemingly allow the organisms continued existence through 



