182 REPRODUCTION 



passing into the body of the mother in one act of pairing, and 

 yet but one or two ova are fertiHzed. Both these figures and 

 those of the fish eggs mentioned above seem to indicate an 

 appalHng waste, yet we must suppose these excessive numbers 

 to be necessary in order to ensure a sufficiency of fertiHzed 

 egg cells. 



There are, however, other methods of fertilization. As a 

 rule the spermatozoa follow in the body of the animal a 

 definite channel ; but there are worm-like animals, Turbel- 

 LARiA, into whose body they are introduced through the skin 

 at any spot. They then have to make their way through the 

 tissues of the body, the muscles, etc., until they reach the 

 eggs. A further modification of this method of fertilization 

 is that in some flat-worms, leeches, and even an animal as 

 high up as Perijjatus, packets, called spermatophores, of sper- 

 matozoa are made up. These packets are implanted in the 

 animal like the arrows in St Sebastian's body. The tip of the 

 implanted spermatophore is then dissolved and the spermato- 

 zoa stream out and force their way through the muscular 

 and connective-tissues and other fabrics of the body until 

 they reach the eggs. 



Packets of spermatozoa or spermatophores are found in 

 other classes, for instance, in the snail, where a hardened mass 

 of mucus whose edges are rolled round so as to form a groove 

 holds the spermatozoa as in a case. This case or spermato- 

 phore is deposited by the snail acting as the male, for the 

 snail is both male and female, in the genital orifice of another 

 snail. In a few days the case of the spermatophore is dissolved 

 and the contained spermatozoa are set free ready to fertilize 

 the ova. In the Sejna also we find cylindrical spermatophores 

 which are not inserted into the ordinary genital orifice, but 

 are deposited on the skin of the female, near the head. When 

 the eggs are laid, the spermatozoa flow out and fertilize them 

 in the water. 



The Egg 



As a rule in both plants and animals the egg is spherical; it 

 may be, and often is, microscopic; but when a large amount 

 of yolk is stored up in it, it may reach gigantic proportions. 



