T. M. SONNEBORN 187 



Life Cycle Differences. Evidence for the existence of a life 

 cycle in Ciliates was first obtained in abundance by Maupas 

 ( 1888, 1889 ) and was subsequently confirmed by many investi- 

 gators. But it was immediately challenged on theoretical grounds 

 by Weismann and eventually on experimental grounds by many 

 investigators. The chief objection grew out of the experimental 

 prolongation of the cycle as cultural conditions were improved 

 and the maintenance of some Ciliates in culture for very long 

 periods without indications of deterioration or ultimate extinction. 

 From my own observations on P. anrelia (Sonneborn, 1954a) and 

 my analysis of the reports of others on other Ciliates, I have con- 

 cluded that there is indeed a life cycle in these organisms but that 

 it may vary greatly in extent and in its manifestations in different 

 species and varieties. These conclusions are well illustrated by 

 the diversities in life cycle among the varieties of P. anrelia. The 

 same fundamental pattern may be seen in the life cycle of all of 

 them, but there is great variation in the duration of the various 

 stages and, in some respects, in the processes that mark them. 



The life cycle, as Maupas discovered, begins with fertilization 

 and proceeds successively with immaturity, maturity, senility, and 

 death. Each of these stages will be taken up in order, and the 

 differences among the varieties will be pointed out. At the start, 

 it is necessary to emphasize that the course of the life cycle de- 

 pends upon the conditions of culture, and especially upon the 

 abundance or scarcity of available food. What happens at a par- 

 ticular stage may be decisively determined by this and by other 

 conditions which will be mentioned later. In order to follow the 

 entire potential cycle, it is necessary to maintain an excess of 

 food at all times, and this is best accomplished by the laborious 

 method of re-isolating single animals daily. On the other hand, 

 since depletion of food is so decisive and since this is doubtless 

 commonly recurrent in nature, to get a more complete picture of 

 the cycle it is also necessary to starve subcultures taken fre- 

 quently from the overfed daily re-isolation lines and to note their 

 responses to starvation. Complete studies of this sort have been 

 made on but few varieties, yet partial information is available 

 on all of the varieties. To the extent that is now possible, all this 



