EGG AND FEMALE ORGANS OF GENERATION. 



547 



Fig. 178. 



oviducts, which are destined to receive the eggs at their internal 

 extremity and convey them to the external generative orifice. The 

 mucous membrane lining the oviducts is also intended to supply 

 certain secretions during the passage of the egg, which are requi- 

 site either to complete its structure, or to provide for the nutrition 

 of the embryo. 



In the frog, for example, the oviduct commences at the upper 

 part of the abdomen, by a rather wide orifice, which communicates 

 directly with the peritoneal cavity. It 

 soon after contracts to a narrow tube, 

 and pursues a zigzag course down the 

 side of the abdomen (Fig. 178), folded 

 upon itself in convolutions, like the 

 small intestine, until it opens, near its 

 fellow of the opposite side, into the 

 " cloaca" or lower part of the intestinal 

 canal. The oviducts present the same 

 general characters with those described 

 above, in nearly all species of reptiles 

 and birds ; though there are some modi- 

 fications, in particular instances, which 

 do not require any special notice. 



The ovaries, as well as the eggs which 

 they contain, undergo at particular sea- 

 sons a periodical development or increase 



r 



in growth. If we examine the female 

 frog in the latter part of summer or the 

 fall, we shall find the ovaries presenting 



the appearance of small clusters of minute and nearly colorless 

 eggs, the smaller of which are perfectly transparent and not over 

 T |^ of an inch in diameter. But in the early spring, when the 

 season of reproduction approaches, the ovaries will be found in- 

 creased to four or five times their former size, and forming large 

 lobulated masses, crowded with dark-colored opaque eggs, measur- 

 ing J 2 of an inch in diameter. At the approach of the generative 

 season, in all the lower animals, a certain number of the eggs, which 

 were previously in an imperfect and inactive condition, begin to 

 increase in size and become somewhat altered in structure. The 

 vitellus more especially, which was before colorless and transparent. 

 becomes granular in texture as well as increased in volume ; and 

 assumes at the same time, in many species of animals, a black. 



F K M A T< E G E * E R A T I r E R " 



GAUS OF FROG. a, a. Ovaries. 



&, 6. oviducts. c , c. Their internal 



