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MANUAL OF ELEMENTARY ZOOLOGY 



place. The male pronucleus of each conjugant passes 

 over into the other and fuses with the female pronucleus 

 of the latter. The body which belonged to each 

 conjugant comes thus to contain a micronucleus of 

 mixed origin. It is, in fact, a zygote. The zygotes separate 

 and are known as exconjugants. Immediately after separa- 

 tion the meganucleus degenerates, splitting up into shreds, 



which disappear. Thus 

 the meganucleus resem- 

 bles in the fact of its 

 mortality the body-cells 

 of the frog, though the 

 body as a whole has 

 the immortality of a 

 germ-cell or an Amaiba. 

 Meanwhile the joint 

 micronucleus of the ex- 

 conjugant undergoes a 

 development whereby 

 nuclei of both kinds are 

 provided. It divides 

 three times successively, 

 so that the body con- 

 tains eight nuclei. After 

 an interval the body 

 divides into two, each 

 half containing four 

 Fig. 83.— A diagram of the behaviour of nuclei, and after a fur- 

 the micro-nuclei during the con- ther interval these halves 

 lugation 01 Paramecium cauaatum. , . . , . , 



The white circles represent the divide, so that there are 

 portions which degenerate. four individuals, each 



with two nuclei, one 

 of which becomes a meganucleus and the other a micro- 

 nucleus. 



The conditions under which conjugation takes place in 

 Paramecium have been, and are still, the subject of much 

 investigation. Many points still remain to be cleared up, 

 but certain results have now been reached. Conjugation 

 generally occurs at the beginning of a falling off in the 

 supply of food after a period of exceptional plenty that has 

 brought about rapid multiplication. Thus it will often take 



