SEXUAL REPRODUCTION. 



1 33 



tion. The generative cell, which consists like the sperm mostly of 

 nucleus with very little directly associated cell-substance, itself divides 

 to form two (or even more) generative nuclei. One of these passes 

 from the pollen-tube to enter into close union with the nucleus of the 

 female cell, with which it fuses to form the double nucleus ruling the 

 forthcoming development. Exceptionally the other generative nucleus 

 may also 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. Fertilization 

 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 accordance with which the pollen-tube bearing 



FlG. 43. — Illustrating the contrast between male and female flowers in the 

 pink campion (Lychnis diurtia). k 



the generative nucleus is marvelously guided to its destination. The 

 differentiation of the generative nucleus, in contrast to the more vege- 

 tative, and the true nuclear union which forms the climax of fertiliza- 

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

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



IV. Fertilization in Animals. — That the sperms were essen- 

 tial to fertilization was a conclusion by no means recognized when 

 those elements were first seen. Gradually, however, the fact was 

 demonstrated, both by experiment and observation. Jacobi (1764) 

 artificially fertilized the ova of salmon and trout with the milt of these 

 forms, and somewhat later the Abbe Spallanzani extended these experi- 

 ments to frogs and even higher animals. Even he, however, believed 

 that the seminal fluid was the essential factor, not the contained sper- 



