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 
