THE PROTOZOA 133 



only two individuals result from the union of two. It is, however, 

 closely bound up with multiplication, since two divisions, resulting 

 in the production of four individuals, must necessarily intervene 

 before the typical nuclear arrangement is restored. The main 

 function seems to be a rejuvenating process, so preventing the extinc- 

 tion of the race owing to exhaustion. It differs from sexual repro- 

 duction in the higher forms in the absence of the formation of visibly 

 differentiated gametes. A further difference is to be seen in the 

 fact that in the higher animals, after one or a limited number of 

 reproductive periods, the parent forms die. In other words, in the 

 Metazoa the somatic or body tissues are perishable and only the 

 germ cells are potentially immortal. In a similar way Paramcecium 

 and Amoeba, too, for that matter, may be regarded as immortal for, 

 excepting accident or disease, death does not appear to come into 

 the life cycle of these animals. 



The conjugation is termed partial because the whole of the two 

 animals does not fuse ; it is merely a fusion of muclear material, 

 and as far as we can see, no fusion of the cytoplasm of the conjugants 

 occurs. The union, too, can hardly be spoken of as fertilisation, as 

 no gametes are produced. The micronuclei are the important 

 factors in the process, and the essential part of conjugation consists 

 in the union of the two nuclei to form the conjugation nucleus. 



Although the nuclei play such an important part, the 

 meganucleus is not concerned at all, and disintegrates during the 

 process, thus remotely recalling the fate of the soma or body of the 

 higher animals. The micronucleus is alone concerned, and to 

 distinguish the parts played by the two nuclei the larger is often 

 termed the tropho-nucleus, since it is concerned with the feeding or 

 trophic functions, and the smaller, the gono-nucleus, since it is 

 related to reproduction. By some authorities they are regarded as 

 one nuclear apparatus in which the trophochromatin is separated 

 from the idiochromatin or reproductive chromatin, while generally, 

 as in Amceba, the two varieties of chromatin are indistinguishably 

 mixed. 



Another interesting point is the elimination of a certain amount 

 of nuclear material from the micronucleus in a way that recalls, as 

 we shall see later, the rejection of nuclear matter from the ovum of 

 the Metazoa preparatory to fertilisation. 



The two nuclei remaining from the divisions of the original 

 micronucleus do not differ in appearance, but because one of them 

 migrates into the other individual in the way the nucleus of the 

 sperm penetrates the ovum it is sometimes called the male pro- 

 nucleus. In the same way the stationary one is distinguished as the 

 female pro-nucleus. The divisions of the micronuclei are of a 



