58 



NEMATODE WORMS. 



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

 ovwm. The female cell is sedentary, the male is active, the 

 latter being usually armed with a whip-like tail resembling 

 in some respects a flagellate 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 spennatozoon 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. 



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

 17). 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 

 takes place by the en- 

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

 without the shell membrane. The ovum on beincr fertilised 

 then commences to go through the process of segmentation, 



Fig. 17. — Embryo and Ova of Sclebostomum 



RUBRUM. 



