688 EGG AND FEMALE ORGANS OF GENERATION. 



ries appear like small clusters of minute and nearly colorless eggs, the 

 smaller of which are perfectly transparent and less than 0.18 millimetre 

 in diameter. But in the early spring, when the season of reproduction 

 approaches, the ovaries increase to four or five times their former size, 

 forming large lobulated masses, crowded with dark-colored opaque 

 eggs, each 2 millimetres in diameter. At the generative season, in all 

 the lower animals, a certain number of eggs, which were previously in 

 an imperfect condition, increase in size and become altered in structure. 

 The vitellus especially, which was before colorless and transparent, be- 

 comes granular and increased in volume ; and it assumes at the same 

 time a black, brown, yellow, or orange color. In the mammalian egg 

 the change consists only in an increase of size arid granulation, without 

 any remarkable alteration of color. 



The eggs, as they ripen in this way, gradually distend the Graafian 

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

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

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

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

 leave the ovaries, and are discharged. Those which are to be expelled 

 at the next generative epoch may be recognized by their greater degree 

 of development ; and in this way, in many animals, the eggs of no less 

 than three different crops may be distinguished in the ovary at once, 

 namely, 1st, those which are perfectly mature and ready to be dis- 

 charged ; 2d, those which are to ripen in the following season ; and 3d, 

 those which are as yet inactive and undeveloped. In most fish and 

 reptiles, as well as in birds, this regular process of the ripening and 

 discharge of eggs takes place but once a year. In different species of 

 quadrupeds it may occur annually, semi-annually, bi-monthly, or even 

 monthly ; but in every instance it returns at regular intervals, and 

 exhibits, therefore, a well-marked periodical character. 



Action of the Oviducts and Female -Generative Passages. In the 

 frog, after the ripening of the eggs and their discharge from the ovarian 

 follicles, they receive an additional investment while passing through 

 the oviducts. At the time of leaving the ovary, the eggs consist simply 



of the dark-colored and granular vitellus, 

 225< inclosed in the vitelline membrane. They 



are received by the inner extremity of 

 the oviducts, and carried downward by 

 the peristaltic movement of these canals, 

 aided by the contraction of the abdominal 

 muscles. During their passage, the mu- 

 cous membrane of the oviduct secretes 



MATUKE FKOGS' EGGS.- a. an albuminous substance, which is de- 

 While still in the ovary, b. After . 



passing through the oviduct. posited m successive layers, forming 



round each egg a thick coating or en- 

 velope (Fig. 225). When the eggs are discharged, this envelope 

 absorbs moisture from the water in which the spawn is deposited, and 



