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OUTLINES OF EVOLUTIONAEY BIOLOGY 



failing powers are restored by conjugation. The conjugation in 

 this case, however, is not quite like that which takes place in 

 the other unicellular organisms which we have been studying, 

 and the term conjugant may be applied, in preference to the 

 term gamete, to the individuals concerned. 



It may easily be observed that the union of the two con- 

 jugants (Fig. 41) is a merely temporary affair. They remain 

 attached together by their mouth-bearing surfaces for a short 



"FiG. 41. Diagram of the Process of Conjugation in Paramoecium. 



gam., gametic nuclei; If., mouth; meg., meganucleus; mic., micronucleus ; mifj. 

 migratory nucleus ; p.b., products of the division of the micronucleus which dis- 

 appear (polar bodies); st., stationary nucleus ; zyg., zygote nucleus. (For further 

 explanation see text.) 



time and then separate again and continue their independent 

 lives. Before separating, however, they evidently undergo some 

 kind of rejuvenescence whereby their vigour and power of multi- 

 plication are completely restored. This is accounted for by the 

 fact that during the time of their union certain complex nuclear 

 processes take place, the net result of which is an exchange of 

 chromatin material between the two conjugants. 



It will be remembered that Paramoecium differs from most 

 Protozoa in the possession of two nuclei, large and small, or 

 meganucleus and micronucleus (Fig. 41, A, meg. and mic.). The 



