320 THE EVOLUTION THEORY 



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duction, as far as can be recognized, than that of dividing into two 

 daughter-nuclei, I cannot regard this designation as suitable ; it 

 obviously originated in the mistaken interpretation, prevalent till 

 very lately, of conjugation, as a ' kind of reproduction,' and this in 

 its turn depends on the conception, transferred from multicellular 

 organisms, of fertilization as a 'sexual reproduction.' We shall 

 immediately see that the micronuclet;s plays the main part in con- 

 jugation, and from this we may suppose that it otherwise fills no role 

 in the life of the animal, and therefore it may best be designated the 

 'supplementary ' or reserve nucleus. In every conjugation the macro- 

 nucleus, which has hitherto been active, breaks up and becomes 

 completely absorbed, very much like a ball of food. This of course 

 takes place ' slowly ; the large nucleus elongates, becomes indented, 

 falls into several pieces, and these are so gradually absorbed that, 

 even after the act of conjugation has been accomplished, irregular 

 fragments of the macronucleus often lie about in the animal 

 (Fig. «5. 9). 



But while the macronucleus falls to pieces the previously minute 

 micronucleus grows enormously and forms a distinct longitudinally 

 striated spindle (i, mi). About the same time these divide in both 

 animals, and each of the daughter-nuclei immediately divides again, 

 so that after these two divisions four spindle-shaped descendants of 

 the micronucleus are to be seen in each animal (Fig. 85, 4). We 

 have previously noted that the apparatus for nuclear division in 

 unicellular ^organisms was similar to that in multicellular organisms, 

 and yet was different from it. In these ciliated infusorians we see 

 an essential difference, for the striated spindle, after the division into 

 daughter-chromosomes has taken place, lengthens out enormously, 

 and becomes so thin in the middle of its length (2) that the two 

 daughter-nuclei at the ends of this long stalk suggest the appearance 

 of a very long and thin dumb-bell, or of a long silk purse. Of asters 

 (centrospheres) there is nothing to be seen, and the mechanism of 

 division is still very obscure ; it almost seems as if a rapidly growing 

 substance forced the two groups of chromosomes apart. 



Hardly have these four descendants of the micronucleus arisen 

 when three of them begin to break up and very shortly disappear; 

 only the fourth is of any further importance, and it divides once 

 more (5), and so gives rise to the two nuclei which play the chief part 

 in the process of conjugation — the copulation-nuclei, exactly analogous 

 to the male and female pronuclei in the fertilized ovum (5, mi*). 

 But in this case each of the two animals functions doubly, that is, 

 both as male and female, for each sends one of the two copulation- 



