CONJUGATION 77 



the other. The process, which is an exceedingly complicated one, has 

 been studied in detail, especially in species of Parameciu7n. In the case 

 of Paramecium putrinum,, for instance, each individual possesses a macro- 

 nucleus and a micronucleus. When two individuals associate in con- 

 jugation, they become closely united by their peristomes and the side of 

 their bodies behind this (Fig. 43). The macronuclei become elongated 

 and undergo a series of divisions till a large number of fragments are 

 produced. All these ultimately degenerate and disappear. Meanwhile, 

 the micronuclei have divided by mitosis, and the two nuclei thus formed 

 in each ciliate again divide by mitosis. At this stage each ciliate or 

 conjugant, as it is called, contains four nuclei and a number of degenerating 

 bodies derived from the macronucleus. Three of the nuclei in each now 

 degenerate, so that each conjugant is left with only one. This one now 

 divides again, and of the two resulting nuclei in each conjugant, which as 

 far as can be seen are exactly alike, one is a stationary nucleus and the 

 other a migratory one. The migratory nucleus of each conjugant now 

 passes over and fuses with the stationary nucleus of the opposite conjugant. 

 The resulting nucleus, which is a zygote nucleus, now divides to give rise 

 to two, these two to give four, and the four to produce eight nuclei. At 

 this stage the ciliates, each of which has eight nuclei and still the remains 

 of the degenerating nuclei, separate from one another and swim away. 

 Of the eight nuclei, four increase in size and become macronuclei, three 

 degenerate, while the remaining one retains its character as a micronucleus. 

 The latter divides to form two micronuclei, and this is followed by division 

 of the ciliate itself in such a manner that two of the macronuclei and one 

 of the micronuclei pass to each daughter ciliate. At the next division of 

 these daughter ciliates the micronucleus divides, and each resulting ciliate 

 receives one of the two macronuclei and one of the tw^o micronuclei. 

 Thus, the nuclear condition of the original ciliate is regained. At all 

 subsequent divisions of the ciliate, both the macro- and micro-nuclei divide. 



A. Two associated conjugants with intact macronuclei and commencing division of micronuclei. 



B. Macronuclei and micronuclei dividing. 



C. Bivided-up macronuclei and two dividing micronuclei in each conjugant. 



D. Three of the four micronuclei in each conjugant have degenerated, while the remainmg one is 



commencmg to divide. 



E. The micronucleus of each conjugant is drawn out into a long spindle. 



F. Four resulting micronuclei near the point of union of the two conjugants. 



G. Union of the micronuclei in jDairs. 



H-L. Progressive division of the micronuclei till each conjugant has eight. The conjugants 

 finally separate (L.) 



M-N. Three micronuclei degenerate, four become macronuclei, while one remains and divides. 

 The ciliate divides. 



0. One product of division of the form with four macronuclei and two micronuclei. It contains 

 two macronuclei and one dividing micronucleus. The ciliate divides. 



P. One product i )f the division of the form with two macronuclei and two micronuclei. It contains 

 one macronucleus and one micronucleus, and thus resembles the ciliates before they com- 

 menced conjugation. 



