PHYLUM PROTOZOA. CILIATES 



63 



Micronucleus and animal 

 divide twice to produce 

 four small paramecia 



Two paramecia unite 

 by their oral grooves 



Macronuclei degenerate, 

 micronuclei divide 



Four micronuclei become macronuclei, 

 three degenerate, one remains 



Three micronuclei degenerate 



OJI 



The fusion micronucleus 

 divides three times 



K 



The animals separate 

 Only one member 

 of the pair is shown 



Remaining micronucleus divides 

 unequally and smaller micro- 

 nuclei are exchanged 



The two micronuclei in each fuse 



Figure 30. Conjugation in Paramecium caudatum. The first stage in conjugation is shown at 

 the top and at the middle of the diagram. Not all stages are shown. In the interest of clarity, 

 the macronuclei are omitted from the third stage; actually they do not disappear completely 

 until after the conjugating animals have separated. 



indicated by the rate of division. The cycle 

 may thus be prolonged by employing a 

 varied culture medium. However, it is now 

 known that this paramecium does undergo 

 a process of self-fertilization called autogamy 

 at regular intervals, but some other ciliates, 

 over a period of years, do not undergo either 

 conjugation or autogamy. Therefore, nuclear 

 reorganization may be essential in some 

 ciliates but not in others. 



Mating types in conjugation 



At one time it was thought that there 

 were no differences between the two para- 

 mecia which join in conjugation, but now 

 it is known that there are mating types 

 within each species. For example, in Parame- 

 cium aurelia at least two mating t\pes 

 ("sexes") occur, designated I and II. Mem- 

 bers of mating type I do not conjugate with 

 each other but will conjugate with members 



