6/2 LECTURE XXIII. 



female pronucleus or that the attraction is mutual, and 

 that it travels towards and fuses with the female pronucleus 

 because it is so attracted. 



In support of his view that no attraction exists between 

 the male and the female pronucleus, Strasburger adduces the 

 well-known fact that nuclear fusion may take place between 

 somatic cells, as, for instance, in the embryo-sac of Angio- 

 sperms ; between nuclei, that is, which might a priori be 

 regarded as quite similar. But this is by no means con- 

 clusive evidence on his side. It is quite clear, in this case as 

 in the preceding, that there must be some force which de- 

 termines the fusion of the nuclei, and it may be that in this 

 case also that force is an attraction existing between the 

 nuclei, an attraction which may be the expression of a quali- 

 tative difference between them. There is another fact which 

 is suggested by these considerations, namely, the production 

 of graft-hybrids, and which may be appropriately considered 

 here. Strasburger has rightly pointed out that the pro- 

 duction of a graft-hybrid, as for instance the Cytisus Adami, 

 probably depends upon a process of nuclear fusion taking 

 place between the cells of the scion and of the stock, a pro- 

 cess which does not usually take place in grafting. If this be 

 so, it is difficult to explain the facts otherwise than by 

 assuming that the nuclear fusion between the scion and the 

 stock only takes place when there exists such a qualitative 

 difference between the nuclei as to determine their coalescence. 

 We adhere, then, to the view that there is a qualitative differ- 

 ence between the nuclei of the corresponding male and female 

 gametes, and that it is upon this difference that their coales- 

 cence depends. 



It seems probable that the difference between a male and 

 a female gamete is brought about in the course of their de- 

 velopment, and it is probably closely connected with the 

 extrusion of the polar body. It may be that the loss of sub- 

 stance is not qualitatively the same in the development of a 

 male and of a female gamete respectively. This view has 

 been stated by Minot and van Beneden in this way, that in 

 the extrusion of the polar body from the developing female 



