38 SEX 



specialised as a locomotor element. Were 

 both loeomotor, as in some Protozoa, the 

 likelihood of cross-fertilisation would be 

 greater still, but the egg would not then be 

 so well adapted in the way of providing the 

 initial material for development and some 

 nutriment as well. As for the condition seen 

 in most Crustaceans and in Nematode worms, 

 where the sperms are sluggish, and amoeboid 

 rather than flagellate, the males have to make 

 up for the loss of activity on the part of their 

 spermatozoa, by directly bringing these into 

 close proximity to the ova. (b) The other 

 advantage of division of labour among the 

 germ-cells is that the one which is to undergo 

 development after fertilisation by the other 

 can be safely endowed with a legacy of yolk, 

 can be ensheathed within envelopes which 

 protect the delicate embryo, or can be hidden 

 away in safe places within the mother's body 

 and there undergo development. Thus, look- 

 ing backward, we may justify as great steps 

 in evolution the specialisation of egg-cells and 

 sperm-cells (see Fig. 5). 



SIGNIFICANCE OF FERTILISATION. In most 

 animals large numbers of ova are produced, 

 and in many cases only a small percentage are 

 fertilised. Inconceivably large numbers of 

 spermatozoa are produced, but all die, except 

 the small minority that reach the ova. In all 

 but a few cases the spermatozoon and the 

 ovum that unite in fertilisation come from 

 different parents ; for although there are many 

 hermaphrodite (bisexual) animals, e. g. snails, 

 earthworms, leeches, pairing none the less 

 occurs, in fact, of two-fold nature, simul- 



