io8 PROTOZOA — THE SIMPLEST ANIMALS. 



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

 has been called a "Plasmodium." 



(2) It is known that more than two individual Gregarines 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 

 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 n 

 " plasmodium " formation. 



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

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

 small free-swimming 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 

 characteristics 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 Para- 

 macium, 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 

 the species is sustained, peculiarities or pathological variations of one 

 individual being counteracted by other characteis 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 

 grandis) were engaged in conjugation, a single individual had, by 

 ordinary asexual division, given rise to a family of from forty thousand 



