86 THE SMALLEST LIVING THINGS 



nucleus derived in the same way. Thus fertilization is mutual, 

 and by the union of a wandering nucleus and a stationary nucleus, 

 each of which has one-half the normal number of chromosomes, 

 the normal number is restored in the fused nucleus (amphi- 

 nucleus). In this way the germ-plasms of two individuals are 

 represented in each amphinucleus — a universal biological phe- 

 nomenon to which the term amphimixis is applied. 



In the meantime and after the two conjugants, now known 

 as ex-conjugants, have separated, the old macronuclei break up 

 into fragments, and these, together with the unused micronuclei 

 resulting from the maturation divisions, are resorbed into the 

 cytoplasm. New macronuclei and new micronuclei are formed 

 by division of the amphinuclei and the characteristics of the 

 species are restored. 



Here, as at division, a complete reorganization of the de- 

 rived structures occurs with conjugation. But the housecleaning 

 is more drastic and involves all of the nuclear substances as 

 well as the external structures. The protoplasm is quite made 

 over, and not only is there resorption and distribution of great 

 quantities of nucleoproteins but the later nuclear make-up is quite 

 different by reason of the fusion (amphimixis) of nuclear parts 

 of two individuals. Again, this process results in a high poten- 

 tial of vitality, the characteristic of youth. 



Encystment and Endomixis 



Like other protozoa, the ciliates have the ability to encyst, 

 and, protected by the impervious cyst membranes, such encysted 

 forms are able to withstand unfavorable conditions of the environ- 

 ment. There is little or no evidence to indicate that encystment 

 is induced by such adverse conditions, although, theoreti- 

 cally, there is no reason why certain conditions of the environ- 

 ment should not bring about the reactions involved in encystment. 

 It is probable that the phenomena of encystment are due to deep- 

 lying causes in the protoplasm itself and occur only when a 

 certain stage of differentiation has been reached. If this is the 

 case, then under adverse conditions those forms which have al- 

 ready encysted will be protected and others will be destroyed. 

 During the process of encystment a ciliate resorbs its external 

 organs, loses its water, and contracts into a dense ball of proto- 



