REPRODUCTION 211 



more vigorous than the parent clones, thus showing rejuvenescence 

 through conjugation; (2) conjugation of young clones results in little 

 or no mortality, while that of old clones results in high (often 100 per 

 cent) mortality; (3) conjugation between a young and an old clone, 

 results in the death of most or all of the exconjugants; (4) the two 

 members of a conjugating pair have the same fate; and (5) what 

 other causes besides age bring about the death, weakness or ab- 

 normality of the exconjugants, are not known. 



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 

 a long chain of generations indefinitely. Jennings (1929, 1942) who 

 reviewed the whole problem states: "Some Protozoa are so con- 

 stituted that they are predestined to decline and death after a 

 number of generations. Some are so constituted that decline occurs, 

 but this is checked or reversed by substitution of reserve parts 

 for those that are exhausted; they can live indefinitely, but are 

 dependent on this substitution. In some the constitution is such 

 that life and multiplication can continue indefinitely without visible 

 substitution of a reserve nucleus for an exhausted one; but whether 

 this is due to the continued substitution, on a minute scale, of re- 

 serve parts for those that are outworn cannot now be positively 

 stated. This perfected condition, in which living itself includes con- 

 tinuously the necessary processes of repair and elimination, is found 

 in some free cells, but not in all." 



