SEXUAL REPRODUCTION 219 
mistakable, and is of considerable import- 
ance in its connection with the sexual act. 
The chief character which urges itself on 
our notice consists in the relatively large 
size of the egg or female gamete, and the 
small size of the other, the male or sperm. 
The egg not only becomes large, but it 
loses the power of independent motility. 
It consists of a bulky mass of cytoplasm, in 
which nutritive matter is often present, and 
it also contains a large and somewhat watery- 
looking nucleus. 
The sperm, on the other hand, is small and 
compact. It is nearly always actively motile, 
though this character is almost or entirely 
abandoned in certain groups, such as the 
highest flowering plants in which this has 
evidently occurred as the result of correlation 
with other secondary changes connected with 
pollination, which render motility useless 
or even disadvantageous. In another im- 
portant respect the sperm also differs from 
the egg, inasmuch as it tends to become 
composed almost entirely of the cell nucleus, 
the cytoplasm being merely represented by 
the cilia and a thin skin which sheaths the 
nucleus as a whole. 
One of the results secured by fertilisation 
has already been. pointed out, namely, 
the vigorous development so characteristic 
of the sexually produced organism. But 
there is another and perhaps hardly less 
important consequence, namely, that the 
