The Biology of Senescence 



divides longitudinally in the normal flagellate manner, while in 

 the other division is transverse, the anterior daughter receiving 

 all the extranuclear organelles of the parent cell except the 

 axostyle, while new organs are formed for the posterior daughter. 

 The axostyle is resorbed (Cleveland, 1938). The possibility that 

 the 'inheritance' of organelles may modify the age status of the 

 inheritor certainly merits re-investigation. 



3 2 The l Senescence' of Clones 



A large part of the literature included in the bibliographies 

 of senescence deals with the presence or absence of 'ageing' in 

 protozoan clones. Maupas (1886) appears to have been the first 

 to draw an analogy between somatic ageing in metazoa and the 

 behaviour of protozoan populations. He predicted that such 

 populations would display a life-cycle including a phase analog- 

 ous to metazoan senescence, and ending in the death of the 

 population, unless nuclear reorganization by conjugation, or 

 some similar mechanism, brought about the 'rejuvenation' of 

 the stock. For many years a vigorous competition was conducted 

 between proto-zoologists in seeing how many asexual genera- 

 tions of Paramecium, Eudorina, and similar creatures they could 

 rear. In the course of this process much nonsense was written 

 about 'potential immortality', but a great deal was learnt about 

 protozoan reproduction and culture methods. It became evi- 

 dent that some clones deteriorate and others, including somatic 

 cells such as fibroblasts in tissue-culture, do not. Calkins (1919) 

 in a classical study showed that strains of Uroleptus mobilis kept 

 in isolation culture without conjugation underwent senescence 

 characterized by falling-off of growth-potential, degeneration 

 of nuclei, and ultimate loss of micro-nucleus. These strains ulti- 

 mately became extinct. Conjugation at any stage of the process, 

 and probably also endomixis, produced an immediate reversion 

 to normal, regardless of whether the conjugates came from old 

 or young isolation strains. Sonneborn (1938) succeeded, by 

 selection of strains of Paramecium in which endomixis was long 

 delayed, in breeding a race which no longer exhibited any kind 

 of nuclear reorganization. These strains invariably died after 

 4 or 5 months. Rizet (1953) has recently reported similar results 



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