142 THE EVOLUTION OF SEX. 



unite with the nucleus of the egg-cell, but this is almost as rare 

 as " polyspermy" among animals. According to Strasburger, the 

 cell-substance of the pollen-grain or pollen-tube which surrounds 

 the nucleus has no direct influence in the essential act. Fer- 

 tilisation is a union of two nuclei, " the cell-substance of the 

 pollen-tube is only the vehicle." He confirms the observations 

 of Pfeffer, as to the reality of an osmotic attraction between at 

 least the surroundings of the two essential elements, in accord- 

 ance with which the pollen-tube bearing the generative nucleus 

 is marvellously guided to its destination. The differentiation 

 of the generative nucleus, in contrast to the more vegetative, and 

 the true nuclear union which forms the climax of fertilisation, 

 are two very important facts, showing the unity of the process 

 not only in higher and lower plants but in all organisms. 



§ 4. Fertilisatio7i in Ani??ials. — That the sperms were essen- 

 tial to fertilisation was a conclusion by no means recognised 

 when those elements were first seen. Gradually, however, the 

 fact was demonstrated, both by experiment and observation. 

 Jacobi (1764) artificially fertilised the ova of salmon and trout 

 with the milt of these forms, and somewhat later the Abbe 

 Spallanzani extended these experiments to frogs and even higher 

 animals. Even he, however, believed that the seminal fluid 

 was the essential factor, not the contained spermatozoa. Through 

 the experiments of Prevost and Dumas (1824), Leuckart (1849), 

 and others, attention was directed to the real import of the 

 sperms, which Kolliker referred to their cellular origin in the 

 testes. The presence of the sperm within the ovum was 

 observed in the rabbit ovum by Martin Barry in 1843 ; by 

 Warneck, in 1850, for the water-snail, a fact confirmed about 

 ten years afterwards by Bischoff and Meissner ; in the frog 

 ovum by Newport (1854) ; and in successive years it was gradu- 

 ally recognised in a great variety of animals. 



The external devices which secure that the sperms shall 

 reach the ova are very varied. Sometimes it seems more a 

 matter of chance than of device, for the sperms from adjacent 

 males may simply be washed into the female, as in sponges and 

 bivalves, with the nutritive water-currents. In other cases, 

 especially well seen in most fishes, the female deposits her 

 unfertilised ova in the water ; the male follows and covers them 

 with spermatozoa. Many may have watched from a bridge the 

 female salmon ploughing along the gravelly river bed depositing 

 her ova, careful to secure a suitable ground, yet not disturbing the 



