ESSENTIAL FEATURES OF SEXUAL PROCESS 1343 



exchange of micro-nuclei, one micro-nucleus from A passing into B, 

 while one micro-nucleus from B passes into A. The two cells then 

 separate, a single micro-nucleus being formed in each by the amalga- 

 mation of the two. This micro-nucleus then divides three times, so 

 that eight nuclei are formed, while the cell itself divides into four, two 

 nuclei passing into each of the daughter cells. Of these one enlarges 



Second fission 



First fission, after separation 



Differentiation of micro- and 

 macro-nuclei 



Separation of the gametes 



Division of the cleavage- 

 nucleus 



Cleavage-nucleus 



Exchange and fusion of the 



germ-nuclei 

 Germ-nuclei 



Formation of the polar bodies 



Union of the gametes 



FIG. 551. Diagram showing the history of the micro -nuclei during the 



conjugation of paramcecium. (From WILSON after MATTPAS.) 



x and Y represent the opposed macro- and micro -nuclei in the two gametes ; 



circles represent degenerating and black dots persisting nuclei. 



to form the macro-nucleus, while the other remains as the micro-nucleus. 

 After conjugation has occurred, the colony of infusoria takes on, so 

 to speak, a new lease of life, and there is a rapid production of new 

 generations by simple division of the cells, in which both macro - 

 nucleus and micro-nucleus take part. Conjugation apparently only 

 occurs in the presence of adverse conditions, and may be prevented 

 almost indefinitely by maintaining the colonies in as favourable condi- 

 tions as possible. In certain organisms, especially in AlgaB, in which 

 similar phenomena take place, each organism after conjugation may 

 surround itself with a thickened wall and remain for a considerable 

 length of time in a state of suspended animation. It is very difficult 

 to understand the advantage of this interchange of nuclear material 



