XII.] CONJUGATION AND SEXUAL REPRODUCTION. l8j 



yet compared the number of the idants in a micronucleus pre- 

 paring for conjugation with the number in a micronucleus of 

 an Infusorian in the act of fission ; and the few figures which 

 we possess, of either of these stages, afford us no reliable 

 information on the point. The figures which Maupas gives 

 of the micronucleus preparatory to conjugation in Pammaeciuni 

 caudatitni and Onychodromiis grandis, lend support to my view, 

 in so far as the number of idants is very large. In the first 

 species I counted twenty-one in the half spindle which is 

 figured, giving a total of about forty-two. But I will not lay 

 too much stress on this point ; the simplicity of m}'- attempted 

 explanation of the changes in the micronucleus appears to me 

 to be strongly in favour of the view upon which the explanation 

 is based. 



If this assumption be well founded it provides a verj' simple 

 solution of the problem of the complex divisions and repeated 

 disintegrations of the micronucleus. The first and second divi- 

 sions are reducing divisions which diminish the previously doubled 

 idants to half the normal number, corresponding exactly to the 

 'reducing divisions' of sperm- and egg- mother-cell. The third 

 division, hozvever, which produces the two reproductive nuclei 

 (male and female), from one of the four grand-daughter-nuclei 

 of the micronucleus, is an ^ equal division' which causes each 

 daughter-nucleus to contain as many idants as were possessed 

 by the parent nucleus. This last division has no analogue in 

 Metazoa, simply because their germ-cells are invariably either 

 male or female, while the Infusorian micronucleus must give 

 rise to both kinds of reproductive nuclei. 



Three out of the four grand-daughter-nuclei of the micro- 

 nucleus disappear, only one dividing to form the reproductive 

 nuclei [D in the diagram, P^ig. XII.). The fact that the others 

 disintegrate can be understood in so far that they are super- 

 fluous and functionless, just like the polar cells of the animal 

 egg. It is more difficult to explain why these three are always 

 present, and still harder to find the true reason, the causa 

 efficiens, of their disintegration. 



With regard to this last question, an observation of Maupas 

 may put us on the right road. He believes that he has observed 

 that, of the four grand-daughter-nuclei derived from the micro- 

 nucleus, the one which lies nearest to the bridge connecting the 



