REPRODUCTION AND GROWTH 175 



strain must be '' rejuvenated " in order that reproduction may 

 again begin. Rejuvenation may be effected by '* conjugation " 

 of pairs of the senescent individuals (see next section). But it 

 can also be effected by the addition of some " stimulant " 

 substance to the water in which the strain lives. In this way 

 reproduction may be continued indefinitely — or senescence may 

 be prevented from occurring. 



But it is well known that wild animals may not reproduce when 

 kept in artificial conditions, in captivity, and we may not, without 

 some reservations, extend the results of laboratory observations 

 to the cases of organisms living naturally " in the wild." And 

 our knowledge of the indefinitely persistent reproduction, by 

 simple division, in the cases of Bacteria, etc., indicate that " sene- 

 scence " must not necessarily be associated with the reproduction 

 of the unicellular organisms. 



63^. Conjugation — the Transition to Sex. When sene- 

 scence occurs in a strain the process of conjugation may occur. 



Gross aspects of conjiigatio7i. Two similar individuals (for 

 instance, two Paramoecia) approximate and their bodies partially 

 coalesce. Then the individuals separate and each of them begins 

 again to divide with full vigour. After a time senescence occurs 

 and is again followed by conjugation and so on. 



Finer aspects of conjugation. The process is variable in detail. 

 In Paramoecium it occurs as follows : there are two " conjugants " 

 and each contains two nuclei (macro- and micro-nuclei). When 

 the conjugants coalesce (or before that, perhaps) one of these nuclei 

 divides several times and two of the daughter nuclei that so arise 

 have special significance — these are the stationary and migrant 

 ones. In conjugation the stationary nucleus remains in its parent- 

 organism, but the migrant one passes over into the body of the 

 other conjugant and fuses with the stationary nucleus of the latter. 

 Thus : 



Fig. 23. — Diagram of the Essential Process in the Division of 



Paramgecium. 



