58 



OVUM. 



received by the open fimbriated extremity of 

 the Fallopian tube. 



The empty ovicapsule now undergoes a 

 remarkable change by the deposit in its inte- 

 rior of the substance termed corpus luteum, 

 the quantity and nature of which vary greatly 

 according as the escape of the ovule is fol- 

 lowed or not by pregnancy. Of this change 

 more will be said hereafter. The result in both 

 cases is the ultimate closure and obliteration 

 of the ovicapsules. 



In birds, scaly reptiles, and cartilaginous 

 fishes, the greater size of the ovules when in 

 a state of maturity is connected with a modi- 

 Fis. 42. 



Relation of the ova to the ovary in Birds. 



A. Ovary of a fowl, showing at a a a the most 

 developed ova hanging from the ovary in their pedi- 

 culated capsules ; the non- vascular bands are seen 

 on their most projecting sides; at bb, the empty 

 capsules or calyces of ova that have been previously 

 discharged ; at c c, the more compact part near the 

 root of the ovary, where the ova are less developed. 



B. Diagrammatic section of one of the most ad- 

 vanced of the capsules ; o, the extended ovarian 

 substance forming the capsule ; p, its pedicle ; c, 

 indicates in this and the preceding figure the most 

 common position of the cicatricula or germ-disc and 

 vesicle; o v, the two layers of the ovicapsule or 

 ovisac, into which the blood-vessels penetrate : the 

 dotted line v m, marks the vitelline membrane. 



fication of the structure of the ovary and the 

 ovicapsules. 



Previous to the age for breeding, the ovary 

 of birds in which animals only one ovary 

 and oviduct is usually developed or attains to 

 functional activity is a solid organ of a less 

 firm texture than that of mammalia, and is 

 adherent to the vertebral column in the mid- 

 dle of the dorsal region. It contains at an 

 earlier period a much greater number of ovi- 

 capsules of a considerable size than are per- 

 ceptible in mammalia. The stroma or ova- 

 rian substance is in less proportion, therefore, 

 to the ovicapsules and ova. 



As the ovules become more developed, 

 they increase rapidly in size ; and we then 

 perceive that they bear a different relation to 

 the ovicapsules from that which has previously 

 been described in mammalia, as they fill com- 

 pletely the ovicapsules, and there is no fluid 

 or loose cellular layer between the outer sur- 

 face of the vitelline membrane and the lining 

 membrane of the theca.* As some of the 

 yolks approach maturity their increase in size 

 is proportionally much greater, and the theca 

 or ovicapsule, and along with it the ovarian 

 substance, is distended in like proportion ; 

 and in this manner the ovary of the fowl at 

 the breeding season has lost its appearance of 

 compactness or solidity, and seems to be 

 composed almost entirely of a larger num- 

 ber of pediculated ovarian capsules of the 

 most various sizes, filled with the yolks or 

 ovules in all stages of development. Never- 

 theless, a little attention shows the solid part 

 of the ovary still remaining at the part at- 

 tached to the vertebral column, forming the 

 ground, as it were', from which the pedicles 

 of the enlarged capsules spring, and contain- 

 ing still a very large number of minute 

 undeveloped ovules in their correspondingly 

 small ovicapsules. 



The large ovicapsules of the developed 

 ovary of the bird consist of two layers, which 

 are loosely united together by blood-vessels 

 and binding tissue towards the pedicle and 

 over the greater part of the capsule, but are 

 more firmly knit together at the free border. 

 At the latter place the blood-vessels of the 

 theca, which are on all the other parts dis- 

 tributed in wide or comparatively large chan- 

 nels very thickly set, suddenly become so 

 small and delicate as to give, at first sight, 

 the appearance of an absence of vascularity 

 in the course of a band of about ^th of an 

 inch in breadth, and extending across a large 

 portion of the free circumference of the cap- 

 sule.f This is the so-called stigma, at which, 

 when the ovule is to escape from the ovary 

 and to be transferred into the oviduct, the 

 rupture of the theca occurs. 



* There may probably be an epithelial lining of 

 this membrane. See H. Meckel's paper, afterwards 

 referred to, Zeitsch. fiir Wissensch. Zool. vol. iii., 

 1852, p. 420. 



f The length of this band or stigma is about 

 equal to the long diameter, or a third of the circum- 

 ference of the capsule at its widest part. It is some- 

 times crossed by a second band of the same kind. 



