The Ovaries 15 



capable of impregnation have all perished through ovulation or 

 other means, the fertiUty of the animal is at an end. 



Since the ova are formed from, specialized peritoneal cells 

 known as the genital epithelium, while the vessels and skeletal 

 tissues are derived from the sub-peritoneal mesoblast we are pre- 

 pared to anticipate the occurrence in the ovary of two somewhat 

 definitely marked zones, the cortex and medulla. The peri- 

 pheral cortex contains the essential ovarian cells, the ova, em- 

 bedded in prolongations from the central or medullary portion, 

 composed of skeletal connective tissue, vessels, nerves and pale 

 muscles. The multiplication of the ova and ovisacs in the cortex 

 causes it to outgrow the medulla and results generally in the 

 formation of a well marked hilus, where the vessels and nerves 

 enter, at which point ova are usually absent. 



The skeletal tissues present an area of increased density at the 

 periphery, where they constitute a firm, dense envelop, the 

 tunica albuginea, over which the peritoneum extends. As the 

 ovisacs develop, they tend to pass centralwards toward the me- 

 dulla, and, continuing to grow as the amount of follicular liquid 

 within them increases, finally occupy the entire thickness of 

 the cortex and, in some species of animals, bulge out beyond the 

 surface as more or less prominent, hemispherical masses or even 

 protrude quite beyond the tunica albuginea as spherical out- 

 growths. As they mature, the external wall of the ovisac atro- 

 phies and finally ruptures at the time of estrum and the ovum is 

 discharged into the pavillion of the oviduct or fimbriated end of 

 the Fallopian tube. (It is claimed that in rare instances in mam- 

 malia the ova are accidently discharged into the peritoneal 

 cavity. ) Usually a Graafian follicle contains but one ovum but 

 in some cases two have been recognized. The total number of 

 ova in the ovaries of our domestic animals has not been deter- 

 mined. In woman, the number has been estimated at about 

 70,000, nearly all of which must undergo retrograde changes and 

 not mature. The ova mature in cycles, varying according to 

 species, climate and degree of domestication. In some well 

 housed and fed domestic animals which are not pregnant, ovula- 

 tion occurs every three or four weeks throughout the year. 



During oestrum the ova are discharged either singly, as in the 

 mare and cow ; two or three, as is frequently observed in the 



