586 



REPRODUCTION. 



FIG. 160. 



until that time, it is one of the elements of the ovarian tissue, and is 

 nourished like any other portion of the female organism. 



Since the ovaries are directly concerned in the production of the egg, 

 they form the essential part of the female generative apparatus ; but 

 in most instances there are also accessory organs, which take part in 

 the process of generation. The most important are two symmetrical 

 tubes, or oviducts, destined to receive the eggs from the ovaries and 

 convey them to the external generative orifice. The mucous mem- 

 brane of the oviducts is usually adapted for 

 supplying certain secretions during the pas- 

 sage of the egg, which complete its forma- 

 tion or provide for the nutrition of the 

 embryo. 



In the frog, the oviduct commences at the 

 upper part of the abdomen, by a rather wide 

 orifice, communicating with the peritoneal 

 cavity. It then contracts to a narrow tube 

 (Fig. 158), folded upon itself in numerous 

 convolutions, until it opens, near its fellow 

 of the opposite side, into the " cloaca " or 

 lower part of the intestinal canal. This is 

 also the general character of the oviducts 

 in nearly all reptiles and birds. 



The ovaries, as well as the eggs which 

 they contain, undergo at certain seasons a 

 periodical development. In the frog, dur- 

 ing the latter part of summer and in autumn, 

 the ovaries appear like clusters of nearly 

 colorless eggs, the smaller of which are per- 

 fectly transparent and less than 0.18 milli- 

 metre in diameter. But in early spring, the ovaries enlarge to four or 

 five times their former size, becoming lobulated masses, crowded with 

 dark-colored opaque eggs, each two millimetres in diameter. At the 

 generative season, in all animals, a certain number of eggs, which 

 were previously imperfect, increase in size and become altered in struct- 

 ure. The vitellus especially, at first colorless and transparent, becomes 

 larger and more granular ; assuming a black, brown, yellow, or orange 

 hue. In the mammalian vitellus the change consists only in an increase 

 of size and granulation, without remarkable alteration of color. 



As the eggs approach maturity, they gradually distend the Graafian 

 follicles and project from the surface of the ovary. When fully ripe, 

 they are discharged by rupture of the follicles, and, passing into the 

 oviducts, are conveyed to the external generative orifice, and there 

 expelled. In successive seasons,. successive crops of eggs enlarge, ripen, 

 leave the ovaries, and are discharged ; and in many animals, the eggs 

 of no less than three different crops may be distinguished in the ovary, 

 namely : 1st, those which are mature and ready to be discharged; 2d, 

 those which are to ripen in the following season ; and 3d, those which 



FEMALE GENERATIVE ORGANS OP 

 FROG. a. a. Ovaries. 6. b. Ovi- 

 ducts, c. c. Their upper orifices. 

 d. Cloaca, showing lower orifices 

 of oviducts. 



