The female organs consist as a rule of two cæcal tubes which unite before opening to the exterior. Each of these is divided into a vagina, uterus, oviduct, and ovary. The ovary constitutes the blind end of the tube, and is formed of a common protoplasmic column, holding a number of nuclei in suspension. The protoplasm becomes cleft around the nuclei in the uppermost part of the tube; the circumscription of the ova proceeds, however, very gradually, and since it commences at the periphery of the column the ova remain attached by stalks to a central axis with one end free. In this way there is formed a rod-like structure known as the _rachis_, which consists of a central axis with a series of half circumscribed ova radiately arranged round it. In the lowest part of the ovary the ova become completely isolated and form separate cells. The protoplasm of the ova, which is clear in the terminal division of the ovary, becomes in most forms filled lower down with yolk-spherules secreted in the body of the ova. These commence to appear at the uppermost extremity of the rachis. In some instances, _e.g._ Cucullanus elegans, yolk-spherules are not formed. In the Oxyuridæ the ova are directly segmented off from the terminal syncytium of protoplasm without the intervention of a rachis; and are therefore formed in the same way as amongst Trematodes, etc. The origin of the membrane around the ova of the Nematoda has been much disputed. At the time when the ovum is detached from the rachis no membrane is present, but it nevertheless appears from Schneider's observations that the region at which it is detached is softer than other parts, so that a kind of micropyle is here formed which disappears after impregnation. A delicate vitelline membrane then appears, around which there is subsequently established an egg-shell, which is usually stated to be formed as a secretion of the walls of the uterus; but Schneider and Leuckart have given strong grounds for believing that it is really a further differentiation of the vitelline membrane due to the activity of the protoplasm of the ovum. The originally single membrane becomes as it thickens split into two layers. The outer of these forms the true egg-shell, and the fertilization of the ovum appears to be a necessary prelude to its production. Round the egg-shell the walls of the uterus often secrete a special albuminous covering. The egg-shell exhibits in many cases peculiar sculpturings as well as terminal prolongations. INSECTA. (49) A. Brandt. _Ueber das Ei u. seine Bildungsstätte._ Leipzig, 1878. (50) T. H. Huxley. "On the agamic reproduction and morphology of Aphis." _Linnean Trans._, Vol. XXII. 1858. _Vide_ also _Manual of Invertebrated Animals_, 1877. (51) R. Leuckart. "Ueber die Micropyle u. den feinern Bau d. Schalenhaut bei den Insecteneiern." Müller's _Archiv_, 1855. (52) Fr. Leydig. _Der Eierstock u. die Samentasche d. Insecten._ Dresden, 1866.