M4 PHYLUM PROTOZOA- -THE SIMPLEST ANIMALS. 



corresponding to fertilisation are of general occurrence. 

 For in many of the Protozoa there occurs at intervals a 

 process of "conjugation" in which two individuals unite 

 either permanently or temporarily. This is an incipiently 

 sexual process ; it is the analogue of the fertilisation of an 

 ovum by a spermatozoon. In many cases, moreover, 

 there is a difference between the two conjugates, analogous 

 to the difference between ovum and spermatozoon. 



(1) It is one of the recurrent phases in the life history of some of the 

 simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 104), that a 

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

 has been called a lt plasmodium" 



(2) It is known that more than two individual Sporozoa and other 

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

 has been applied. 



(3) Commonest, however, is the union of two apparently similar in- 

 dividuals, either permanently, so that the two fuse into one, or tem- 

 porarily, so that an exchange of material is effected. Permanent 

 conjugation has been observed in several Rhizopods, Infusorians, and 

 Sporozoa. Temporary conjugation is well known in not a few ciliated 

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

 Sporozoa is of the same nature, or it may be of the nature of a 

 " plasmodium " formation. 



(4) There are some cases where one of the conjugating individuals 

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

 free-swimming form unites and fuses completely with a stalked indivi- 

 dual of normal size. This "dimorphic conjugation" is evidently 

 analogous to the fertilisation of a passive ovum by an active sper- 

 matozoon. 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 Paraina:ciitin, 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 micronucleus lies by the side 

 of the larger 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 micronucleus 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 micronucleus 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 



