THE OVUM. 



47 



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 peri- 

 toneal 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 peri- 

 toneal investment. Each ovarian tube (fig. 17) consists of three 

 sections: (i) a terminal thread, 

 (2) the terminal chamber or ger- 

 mogen, (3) the egg-tube proper. 



The whole egg-tube is invested 

 in a structureless tunica propria. 



The terminal threads are fine prolon- 

 gations 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 ger- 

 mogen 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 epithe- 



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 Lub- 

 bock.) 



o. ovarian segment, formed of an 

 ovum a, and a mass of yolk-cells, b. 



