132 



AN INTRODUCTION TO ZOOLOGY 



The macronucleus takes no part in these changes at all, but 

 remains unaltered. After, the separation, or perhaps before, 

 however, it breaks up into smaller and smaller fragments which are 

 gradually absorbed and play no further part in the activities of the 

 cell. 



The mouth which has disappeared in both individuals is now 



re-formed, and so each ex- 

 conjugant consists of a more 

 or less normal free-swim- 

 ming and feeding Para- 

 mcecium, save that it only 

 possesses one nucleus, the 

 conjugation nucleus. This 

 nucleus undergoes three suc- 

 cessive divisions, giving rise 

 to eight nuclei, four of 

 which migrate to the ante- 

 rior and four to the posterior 

 end of the animal. Those 

 at the anterior end grow 

 and turn into macronuclei, 

 while of those at the other 

 end, three disappear alto- 

 gether and the fourth re- 

 mains as the micronucleus. 

 The ex-conjugant, a form 

 with five nuclei, is now 

 ready to undergo fission, 

 which it soon does if con- 

 ditions are favourable. The 

 micronucleus divides indi- 

 rectly and the macronuclei 

 are distributed equally to 

 each daughter individual, 

 thus leaving each with 

 three nuclei. Another simi- 

 lar division follows, so that 

 the ex-conjugant gives rise 

 to four daughter individuals 



before the ordinary arrangement of one micro- and one macronucleus 

 is restored, and these then serve as starting points for new series 

 of asexual divisions. 



From the description that has just been given it will be seen that 

 conjugation is not strictly speaking a method of reproduction, because 



FIG. 43. Diagram of the nuclear changes 

 leading to the production of four typical 

 Paramcecia from an ex-conjugant. The 

 position and relative size of the nuclei 

 are purely diagrammatic. 



A., ex-conjugant after the first division of theyion- 

 jugation nucleus ; B.-E., normal Paramaecia. 



