XII.] CONJUGATION AND SEXUAL REPRODUCTION. l8l 



term will soon be apparent. If we next consider the pheno- 

 mena which have been directly observed, we find that, in spite 

 of the alread}'^ mentioned fundamental agreement between the 

 two forms of amphimixis (conjugation and fertilization), there 

 are some not unimportant differences between them. 



This is partly due to the fact that those Infusoria which have 

 supplied the most familiar examples of conjugation, possess two 

 kinds of nuclei, the macronucleus and the micronucleus. To 

 the former is attributed the vegetative functions, while the latter 

 has been termed the 'generative nucleus.' It is certain that 

 both nuclei proceed from the same material, viz. from the 

 combination-nucleus of the animals after conjugation, that is, 

 from a germ-nucleus. It is thus established that their differen- 

 tiation depends on the principle of division of labour, and 

 Maupas probably comes near the truth when he attributes to 

 the macronucleus a ' bon fonctionnement des organes de la vie 

 vegetative et a la forme individuelle,' — a conception which does 

 not precisely coincide with that of Butschli, Gruber, and Hert- 

 wig, who regard it as an * assimilative nucleus' only. Ascer- 

 tained facts indicate that the micronucleus, in the first place, 

 sub-serves amphimixis ; for it and it alone produces the re- 

 productive nuclei. But we must beware of restricting its 

 activity to this single function. Numerous facts tend to show 

 that it has another function, in addition to that which relates to 

 the periods of conjugation. In many species there is not one 

 micronucleus, but two of them, which are found regularly 

 through the whole period of fission, although onl}^ one takes 

 part in conjugation, while the other disintegrates. In other 

 species numerous micronuclei exist, and in Steutor Roeselii there 

 are eight-and-twenty regularly distributed through the whole 

 animal. This indicates that during the period of multiplication 

 of the Infusorian its mass of idioplasm must be greater than 

 during the period of conjugation, and this again points to some 

 special activity during the former period. I do not know of 

 what kind this activity is, and do not care to speculate, since 

 the question has no bearing upon our present subject. This 

 much, however, is determined, that as regards conjugation, the 

 micronuclei bring about the continuity of the gcnn-plasni. Among 

 the Metazoa this continuity is not, in many cases, effected so 

 directly and visibly, but it is brought about, as I believe, by 



