REPRODUCTION. 671 



explain the facts of conjugation. It is impossible to explain 

 why it is that, in Ulothrix, only those planogametes coalesce 

 which have been derived from different cells, that in Aceta- 

 bularia only those planogametes coalesce which have been 

 developed in different resting-spores, that in Dasycladus only 

 those planogametes coalesce which have been produced by 

 different individuals, and many other similar cases might be 

 mentioned, otherwise than on the assumption of a difference 

 of sex. 



From the facts of the development of the gametes it is 

 probable that the sexual diversity depends upon a difference 

 in the constitution of the nuclei. The coalescence of two 

 appropriate gametes may be regarded as taking place in con- 

 sequence of an attraction existing between them, an attrac- 

 tion which may be attributed to the difference between their 

 nuclei. From this point of view, sexual affinity becomes in- 

 telligible. The fertility of the union between two gametes 

 will depend upon the relation between their nuclei : when the 

 nuclei are exactly complementary, the union will be fully 

 fertile : when the relation is less perfect the union will be less 

 fertile : when there is no relation, no union will take place. 

 In illustration of this, reference may be made to the facts 

 already mentioned, concerning the prepotence of some pollen 

 over others, the relative fertility of fertilisation by pollen from 

 various sources, etc. 



Strasburger denies that any such attraction exists between 

 the nuclei of coalescing gametes, but some of his own facts 

 tend to prove its existence. In describing the fusion of the 

 male and female pronuclei in the oosphere of Phanerogams, 

 he speaks of the male pronucleus as being passively conveyed 

 by the cytoplasm to the female pronucleus. But the question 

 at once arises, why does the cytoplasm convey the male pro- 

 nucleus in just such a direction that it meets the female 

 pronucleus ? Even assuming that Strasburger's statement of 

 the case is correct, it must be admitted that it is under the 

 influence of the female pronucleus that the cytoplasm con- 

 veys the male pronucleus to it. But it is probably nearer the 

 truth to say that the male pronucleus is attracted by the 



