54 



CILIATE PROTOZOA 



tT?vtt9T fj.f<f 



after a further interval these halves divide, so that there are 

 four indivitluals, each with two nuclei, one of which becomes 

 a nieganucleus and the other a micronucleus. 



The conditions under which conjugation takes place in 

 raramccium have been, and are still, the subject of much investi- 

 gation. 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 place in an infusion 

 in which the bacteria, having used up 

 the nourishment provided by the 

 plant-remains, are falling off in 

 numbers, and thus the animals, after 

 a plentiful supply of food, are begin- 

 ning to experience dearth. It has been 

 said that descendants of the same 

 exconjugant will not conjugate, and 

 that individuals from another stock 

 must be introduced, but this has been 

 disproved ; generally there seem to 

 be two mating types (they cannot be 

 called sexes, since both are herma- 

 phrodite) which will conjugate with 

 each other but not amongst them- 

 selves. There are some races in which 

 it is difficult to bring about conjuga- 

 tion, others in which it has never 

 been seen, and yet others in which it takes place at short intervals 

 without apparent cause. It seems that each clone, that is, the 

 population derived by asexual reproduction from a single parent, 

 has its own life-history. At first it is sexually immature, and 

 this phase lasts in P. bursaria for several months. Then, for 

 years there is sexual maturity, and conjugation readily takes 

 place. Finally the stock seems to age, and may die out. The 

 species is apparently maintained by occasional very vigorous 

 products of conjugation. Many exconjugants die at once, or after 

 a few divisions. 



Paramecium also has a type of pseudo-sexual process known as 

 endomixis. In this much the same nuclear changes go on as in 



Fig. 28. — A diagram of the 

 behaviour of the micronuclei 

 during the conjugation of 

 Paramecium caxidatitm. The 

 white circles represent the 

 portions which degenerate. 



