XII.] CONJUGATION AND SEXUAL REPRODUCTION. I 79 



figure 7, the fusion of the male and female nuclei to form the 

 germ-nucleus, Kk. 



The essential part of this process is shown even more clearl}- 

 in the annexed diagrammatic representation of the changes 

 undergone by the micronucleus, which Maupas has constructed 

 for Colpidium truncatum. Fig. XII. illustrates diagrammatical!}- 

 the nuclear changes of two conjugating individuals of this 

 species. The black spheres represent the persistent nuclei, 

 while the circles stand for those which disintegrate and dis- 

 appear. Similar processes take place in each individual of the 

 conjugating pair. The micronucleus first grows from its pre- 

 vious small size, A'^, to 3. considerable bulk, and it is shown in 

 A^ as ready for the first fission, producing the two nuclei (B). 

 Each of these daughter-nuclei again divides, and thus the four 

 grand-daughter-nuclei arise (C). Three of these disintegrate 

 and disappear, while one divides and produces two nuclei (D) 

 comparable with the sperm- and egg-nuclei of Metazoa. We 

 may call these the male and female reproductive nuclei, and may 

 regard that as the male which leaves the animal in which it had 

 its birth and enters the other organism in order to fuse with 

 its female reproductive nucleus. This fusion, represented at E 

 in the diagram, leads to the production of the ' combination 

 nucleus \' the analogue of the 'germ-nucleus' of fertilization. 



The old macronucleus disintegrates and is absorbed, but by 

 the double division of the ' combination-nucleus ' two new macro- 

 and two new micronuclei arise, preliminary to the first fission 

 of the whole animal which now commences. 



The essential part of the whole process is the fusion of two 

 equal amounts of nuclear substance, the one derived from one 

 animal and the other from another, and the formation from this 

 nuclear substance, thus derived from two individuals, of the 

 nuclei which dominate the animals after conjugation. This 

 harmonizes with the process of fertilization in that here also 

 two equal masses of nuclear substance, derived from two dif- 

 ferent individuals, unite to form the new germ-nucleus. Now 

 that we at length recognize that the ' nuclear substance ' is the 

 ruling principle of the cell, that Nageli's ' idioplasm ' is the 



1 By this term I mean a nucleus which has arisen by amphimixis, and 

 consists of equal amounts of idioplasm from two individuals. 



N 2 



