112 PROTOZOA. 



It is one of the recurrent phases in the life history of some of the 

 simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 98), that a 

 number of amoeboid units flow together into a composite mass, which 

 has been called a " plasmodium" 



It is known that more than two individual Gregarines and other 

 forms occasionally unite. To this the term " multiple conjugation " has 

 been applied. 



Commonest, however, is the union of two apparently similar individ- 

 uals, either permanently so that the two fuse into one, or temporarily so 

 that an exchange of material is effected. Permanent conjugation has 

 been observed in several Rhizopods, Infusorians, and Gregarines. 

 Temporary conjugation is well known in not a few ciliated Infusorians, 

 and it is possible that a curious end-to-end union of certain Gregarines 

 is of the same nature, or it may be of the nature of a " plasmodium " 

 formation. 



Fourthly, there are some cases where one of the conjugating individ- 

 uals is larger and less active than the other. Thus in Vorticella, a 

 small free swimmin'g form unites and fuses completely with a stalked 

 individual of normal size. To call this "dimorphic conjugation " is 

 hardly necessary, since it is evidently equivalent to the fertilisation of a 

 passive ovum by an active spermatozoon, one of the well-known charac- 

 teristics of reproduction in the Metazoa. 



In Volvox this is even more obvious, for the small and active cells, 

 both in shape and method of formation, recall the spermatozoa of 

 higher forms. The conjugation of ciliated Infusorians, such as ParamtE- 

 duni, has been studied with great care by Gruber, Maupas, R. Hertwig, 

 and others, and though their results are not quite harmonious, the main 

 facts are secure. In many ciliated Infusorians there are two nuclear 

 bodies, one large, the other small. The smaller or micronucleus lies 

 by the side of the larger or macronucleus. The micronucleus divides 

 into parts, while the macronucleus degenerates. Two individual 

 Infusorians (A and B) lie side by side in close contact, a portion of the 

 micronucleus of A passes into B, and fuses with a portion of the micro- 

 nucleus of B, similarly a portion of the micronucleus of B passes into 

 A, and fuses with a portion of the micronucleus of A. In short, mutual 

 fertilisation occurs, the conjugating individuals separate, a new micro- 

 nucleus and a new macronucleus are established in each. 



The precise interpretation of the process is to some extent a matter of 

 mere opinion. We may regard it as a mutual rejuvenescence, each 

 unit supplying some substances or qualities which the other lacks ; or 

 we may regard it rather as a process by which the average character of 

 the species is sustained, peculiarities or pathological variations of one 

 individual being counteracted by other characters in the neighbour 

 (apparently no near relation) with which it conjugates ; or we may see 

 in it a source of variation as the result of new combinations among the 

 essential hereditary substances. The researches of M. Maupas have 

 thrown much light on the facts, and some of his results deserve 

 summary. 



It has been often alleged that the subsequent dividing is accelerated 

 by conjugation ; but Maupas finds that this is by no means the case. 

 The reverse in fact is true. While a pair of Infusorians ( Onychodromus 



