ESSENTIAL FEATURES OF SEXUAL PROCESS 



1201 



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

 indefinitely by maintaining the colonies in as favourable conditions as 

 possible. In certain organisms, especially in Algae, in which similar pheno- 

 mena 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 either to the individual or to the race. 



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. 553. Diagram showing the history of the micro-nuclei during the 



conjugation of paramcecium. (From WILSON after MATJP AS.) 



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



circles represent degenerating and black dots persisting nuclei. 



It has been suggested that as soon as each individual concerned in the pro- 

 cess receives the nuclear material from organisms which may have been 

 exposed to slightly different circumstances, corresponding changes will be 

 introduced into the tendencies to growth of the product of the union. Some 

 of these tendencies may be more advantageous than before, while others may 

 be the reverse. Increased possibility of variation is, however, intro- 

 duced by this admixture of nuclear material, and this may be the advantage 

 of the process to the race. It should be noted that the half of the nucleus 

 lost by each conjugating organism is qualitatively different from that which 

 it retains and probably from that which it receives. A gamete in which 

 the nucleus can be represented by ab, and which by simple division will 

 produce similar organisms with nucleus ab, conjugates with an organism 



