548 EGG AND FEMALE ORGANS OF GENERATION. 



brown, yellow, or orange color. In the human subject, however, 

 the change consists only in an increase of size and granulation, 

 without any remarkable alteration of color. 



The eggs, as they ripen in this way, becoming enlarged and 

 changed in texture, gradually distend the Graafian follicles and 

 project from the surface of the ovary. At last, when fully ripe, 

 they are discharged by a rupture of the walls of the follicles, and, 

 passing into the oviducts, are conveyed by them to the external 

 generative orifice, and there expelled. In this way, as successive 

 seasons come round, successive crops of eggs enlarge, ripen, leave 

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

 at the next generative epoch may always 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 recongized in 

 the ovary at once, viz., 1st, those which are perfectly mature and 

 ready to be discharged ; 2d, those which are to ripen in the follow- 

 ing season; and 3d, those which are as yet altogether inactive and 

 undeveloped. In most fish and reptiles, as well as in birds, this 

 regular process of maturation and discharge of eggs takes place 

 but once a year. In different species of quadrupeds it may take 

 place annually, semi-annually, bi-monthly, or even monthly; but 

 in every instance it recurs at regular intervals, and exhibits accord- 

 ingly, in a marked degree, the periodic character which we have 

 seen to belong to most of the other vital phenomena. 



Action of the Oviducts and Female Generative Passages. In frogs 

 and lizards, the ripening and discharge of the eggs take place, as 

 above mentioned in the early spring. At the time of leaving the 



ovary, the eggs consist simply of the 

 'S- 1 ' 9 - dark-colored and granular vitellus, 



inclosed in the vitelline membrane. 



They are then received by the inner 

 <* A rM extremity of the oviducts, and carried 



I 



downward by the peristaltic move- 

 ment of these canals, aided by the 

 FROGS' Eoosrr. while more powerful contraction of the 



still in the ovary. 1. After passing v j i -\ -TV j.1 



through th oviduct. abdominal muscles. During the pas- 



sage of the eggs, moreover, the mucous 



membrane of the oviduct secretes a colorless, viscid, albuminoid 

 substance, which is deposited in successive layers round each egg, 

 forming a thick and tenacious coating or envelope. (Fig. 179.) 

 When the eggs are finally discharged, this albuminoid matter 



