NEMATODE WORMS. 65 



or gen-m cell , the resvilt of the fertilisation — that is, the entry of 

 the male element into the female — is the production of a fertile 

 ovum. The female cell is sedentarj', the male is active, the 

 latter being usually armed with a whip-like tail resembling 

 in some respects a tiagellate protozoan, but the whip pushes the 

 flagellata, whilst it pulls the spermatozoa. The male cell enters 

 the female cell and then loses its flagellum. On the entry 

 of the spermatozoon into the germ cell, the former travels 

 through the latter until it unites, or rather its nucleus unites, 

 with the nucleus of the female cell, or, as it is more correctly 

 called, the female pronucleus. One spermatozoon alone enters : 

 the entrance takes place through an aperture in the germ cell 

 called the micropyle. At the same time a prominence appears 

 on the ovum, called the "polar prominence,'' which gradually 

 grows out and splits off into two round bodies, the " polar 

 bodies," which pass away and are lost. The result is that a 

 fertile ovum is produced, capable of developing into an in- 

 dividual similar to the male and female parent form, and yet 

 sufficiently plastic to undergo slight variations which may 

 become permanent characters, and so eventually mould a new 

 species. An asexual egg passes out only one, "polar body/' a 

 true ovum two. 



Development of Nematodes. — Nematodes chiefly lay eggs (fig. 

 20). Living young are produced by a few species, when they 

 are called viviparous, to 

 distinguish them from 

 the egg-laying or ovipar- 

 ous species. The ova 

 possess a shell, but in 

 the Trichince this is lost 

 whilst in the mother 

 worm Fertilisation fig. 20.— embryo and ova of Sclerostomdm 



RUBRUM. (Greatly onlarsed.) 



takes place by the en- 

 trance of a spermatozoon into the female cell while it is still 

 without the shell membrane. The ovum on being fertilised 



E 



