52 CILIATE PROTOZOA 



is Split into two, each half containing a daughter nucleus of 

 each kind and one of the contractile vacuoles. The two bodies 

 formed by this fission are, like those of Amoeba, asexually pro- 

 duced young, analogous to the buds of certain higher animals 

 of which we shall speak in a later chapter (p. loi). Their develop- 

 ment involves not only growth but also the remodelling of the 

 body, since each of them lacks half the outward organs of the 

 parent, while those which it has are too large for it. In a well-fed 

 culture, division takes place two or three times a day, but if 

 the animals be ill-nourished it is much less frequent, and if they 

 be starved thev cease to divide. 



CONJUGATION 



The conjugation of Paramecium is a remarkable process, of 

 a kind found only in this creature and in the other members of 

 its class (Figs. 27-29). As a rule, the process begins during 

 the late hours of the night and lasts till the next afternoon. 

 The details differ in different species, but the following is the 

 usual course of events in P. caudatum. Two individuals, which 

 we will call conjugants, come together as those of Monocystis do, 

 but without encysting, and lie with their ventral sides touching, 

 the endoplasms becoming continuous in the region of the gullets, 

 which degenerate, We may compare this with coition. The micro- 

 nucleus of each conjugant leaves its normal position, lies free in 

 the cytoplasm, and grows larger. It then divides twice, and three 

 of its four products degenerate. During these divisions the 

 number of chromosomes is halved, as it is in the gametogenesis 

 of higher animals, though the details of the process differ in the 

 two cases. The remaining micronucleus divides again, this time 

 somewhat unequally, the smaller product being the male pro- 

 nucleus, the larger the female pronucleus. At this stage we may 

 regard each conjugant as containing two gametes, the male, 

 represented by the pronucleus, the female by the pronucleus plus 

 the cytoplasm of the conjugant. These are analogous to a sperma- 

 tozoon and an ovum, so that the animal may be said to be 

 hermaphrodite. The true syngamy now takes place. The m^ale 

 pronucleus of each conjugant passes over into the other and fuses 

 with the female pronucleus of the latter and there is also some 

 mixture of cytoplasm. The body which belonged to each conjugant 

 comes thus to contain a micronucleus of mixed origin. It is, in 



