SEXUAL REPRODUCTION 221 



stance was fraught with tremendous conse- 

 quences which were destined to influence the 

 course of evolution of the entire organic 

 world, of animals no less than plants. 



One of the most singular features of the 

 sexual act, in so far as it can be actually 

 observed, consists in the attraction which 

 the gametes exercise on each other. It is 

 by this means that fertilisation is rendered 

 possible, and is definitely secured. 



As the differentiation of the male and female 

 gametes becomes more pronounced, the im- 

 mobile egg is ardently sought by the motile 

 sperms, and the latter are evidently stimulated 

 by something which emanates from the egg. 

 Even when the sperms are not themselves 

 vigorously motile, they are often, as in the 

 case of the flowering plants, conducted to 

 the egg in an analogous, though more indirect, 

 method in which attraction plays a part. 

 For the pollen tube, in which the male gametes 

 are contained, grows into the cavity of the 

 ovary, and thence to the ovule in which the 

 egg is formed, and it there discharges them in 

 such a way as to render fertilisation almost 

 inevitable. 



But it is simpler to choose a less specialised 

 type than the flowering plant in order to 

 become familiar with the essential facts of 

 fertilisation. For this purpose some of the 

 brown seaweeds (Fucus) afford admirable 

 material. They produce large quantities of 

 eggs and sperms in little conceptacles situated 



