. A text-book of agricultural zoology. Zoology, Economic. 58 NEMATODE WORMS. the male element into the female—is the production of a fertile ovum. The female cell is sedentary, the male is active, the latter being usually armed with a whip-like taU 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 ilagellum. 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 f
. A text-book of agricultural zoology. Zoology, Economic. 58 NEMATODE WORMS. the male element into the female—is the production of a fertile ovum. The female cell is sedentary, the male is active, the latter being usually armed with a whip-like taU 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 ilagellum. 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 fertUe 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. 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 Fia. 17.—Embryo AND Ova OF ScLERORTOMUM .,.,««,« Tr««4-,-i: „„ii iHiBEDM. worm. J? ertilisation 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 then commences to go through the process of segmentation,. Please note that these images are extracted from sc
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1899
