THE FEMALE ORGANS OF GENERATION. 587 



are yet inactive and undeveloped. In most fish and reptiles, as well 

 as in birds, the ripening and discharge of eggs takes place once a year. 

 In different quadrupeds it occurs annually, semi-annually, bi-monthly, 

 or even monthly ; but in every instance it is periodical, returning at 

 regular intervals. 



Action of the Oviducts and other Generative Passages. In the frog, 

 after the ripening of the eggs and their discharge from the ovarian fol- 

 licles, they receive an additional investment in the oviducts. On leav- 

 ing the ovary, they consist only of the dark-colored granular vitellus, 

 inclosed in its vitelline membrane. Their 

 passage through the oviduct is effected by FIG. 161. 



the peristaltic contraction of its walls, "TN 



aided by the abdominal muscles. During r ^^ 



this passage, an albuminous substance, ^*- ^ ^ 



secreted by the oviduct, is deposited round 

 each egg in successive layers, forming a 

 thick envelope (Fig. 161). When the eggs 

 are discharged, it absorbs moisture from 

 the water in which they are deposited, and through the oviduct, 

 swells into a transparent gelatinous mass, 



in which the eggs are imbedded. By its subsequent liquefaction and 

 absorption, it supplies material, for the nourishment of the embryo. 



In scaly reptiles and in birds, the oviducts perform a more import- 

 ant function. In the common fowl, the ovary consists of follicles of 

 various size, loosely united by connective tissue, and containing eggs 

 in different stages of development (Fig. 162, a). As the egg which is 

 approaching maturity enlarges, it distends its follicle, and projects from 

 the general surface of the ovary ; so that it hangs at last into the peri- 

 toneal cavity, retained only by the attenuated wall of the follicle, and 

 a slender pedicle containing its blood-vessels. A rupture of the follicle 

 then occurs at its most prominent part, and the egg is discharged from 

 the lacerated opening. 



As the egg leaves the ovary, it consists of a large, globular, orange- 

 colored vitellus, or "yolk," inclosed in a thin and transparent vitelline 

 membrane. Immediately beneath the vitelline membrane, on the sur- 

 face of the vitellus, is a round white spot, consisting of a layer of 

 minute granules, termed the "cicatricula," in which the ge.rminative 

 vesicle was previously imbedded. At this time the germinative vesicle 

 has usually disappeared ; but the cicatricula is still an important part 

 of the vitellus, and marks the spot from which the embryo begins its 

 development. 



From the surface of the ovary, the yolk projects into the orifice of the 

 oviduct ; and when discharged from its follicle, it is embraced by the 

 expanded upper extremity of this tube, and commences its downward 

 passage. In the fowl, the muscular coat of the oviduct is highly 

 developed, and its peristaltic contractions urge the egg from above 

 downward, somewhat as the oesophagus or the intestine transports the 



