(53) Lubbock. "The ova and pseudova of Insects." _Phil. Trans._ 1859. (54) Stein. _Die weiblichen Geschlechtsorgane d. Käfer._ Berlin, 1847. [Conf. also Claus, Landois, Weismann, Ludwig (No. 4).] The ovum of Insects has formed the subject of numerous investigations, and has played an important part in the controversies on the nature of the ovum. The ovaries are paired organs, rarely directly connected, each consisting of more or fewer ovarian tubes which open into a common oviduct. The oviducts unite into a vagina, usually provided with a spermatheca and accessory glands, which need not be further alluded to. Each ovary is invested by a peritoneal covering, which assumes various characters, and either forms a loose network covering the whole or a special tunic round each egg-tube. It is continuous with the general peritoneal investment. Each ovarian tube (fig. 17) consists of three sections: (1) a terminal thread, (2) the terminal chamber or germogen, (3) the egg-tube proper. [FIG. 17. _A._ OVARIAN TUBE OF THE FLEA, PULEX IRRITANS. (From Gegenbaur, after Lubbock.) _o._ ovum. _g._ germinal vesicle. _B._ OVARIAN TUBE OF A BEETLE, CARABUS VIOLACEUS. (After Lubbock.) _o._ ovarian segment, formed of an ovum _a_, and a mass of yolk cells, _b_.] The whole egg-tube is invested in a structureless tunica propria. The terminal threads are fine prolongations of the ends of the egg-tubes usually continued close up to the heart. At their extremities they frequently anastomose, or even unite into a common thread. In some cases they are absent. They form either direct continuations of the germogen and have the same histological structure, or in other cases are simply prolongations of the tunica propria, and serve as ligaments. The germogen usually consists of two parts: an upper, filled with nuclei imbedded in protoplasm, and a lower, in which distinct cells have become differentiated. The lower part of the egg-tubes is filled with ova which advance in development towards the oviduct, and lie in chambers more or less distinctly constricted from each other. In these chambers there are in most forms in addition to the true ova a certain number of nutritive cells. The true egg-tubes are moreover lined by an epithelial layer which passes in and forms more or less complete septa between the successive chambers. The points which have been especially controverted are (1) the relation of the ovum to the germogen, and (2) the relation of the nutritive or yolk cells to the ovum. To the controversies on these points it will only be possible to give a passing allusion. As has been already hinted there are two distinct types of ovaries, viz. those without the so-called nutritive or yolk cells and those with them[23]. [23] For a list of the genera with and without nutritive cells, _vide_ Brandt, pp. 47 and 48.