Ovulation 47 



rupture of the ovisac occurs with the escape of the ovum into 

 the pavillion of the tube and, following this, while the egg is 

 moving along the oviduct, before impregnation has taken place, 

 the second polar body is supposed to be cast off. 



The exact relation of ovulation to estrum and menstruation 

 has not been determined with complete satisfaction, and conflic- 

 ting views are held. In my personal experience in spaying 

 cows, it has been constantly found that one in estrum has, in one 

 ovary, a ripe Graafian follicle which is ready to rupture and usu- 

 ally does so on being removed, while, if estrum has occurred on the 

 previous day, a fresh corpus luteum is present. This indicates 

 that, for the cow, estrum normally signifies the presence in the 

 ovary of a mature Graafian follicle, about to rupture, and that 

 copulation with the male usually takes place prior to the dis- 

 charge of the ovum from its ovisac. Later, menstruation may 

 occur if impregnation has not taken place. 



Observations on rabbits are similar. When young are born, 

 there already exists, in the ovaries of the doe, a crop of ovisacs, 

 fully matured and ready to rupture. Estrum follows at once 

 upon the birth, copulation occurs, and it is not until after eight 

 to twelve hours have elapsed that the ovisacs rupture and dis- 

 charge their contents into the pavillion of the oviduct, there to 

 become fertilized by the male cells already present. Such is 

 probably in a large measure the rule with other animals. There 

 are reasons for believing that ovulation is favored and at times 

 possibly hastened by coition, but there is no definite relation and 

 the ovulation regularly occurs in the absence of sexual contact. 



Ovulation occurs in more or less fixed cycles and is modified 

 somewhat by climate, season, food and housing. In the mare it 

 usually takes place at intervals of about four weeks during spring 

 and early summer, is less frequent and more irregular during 

 autumn and ;nay be quite absent during winter, except she be 

 well fed and housed. The cow ovulates with considerable regu- 

 larity about every three weeks and, if well housed, the cycle is 

 not liable to interruption during the winter months. 



In the ewe, ovulation apparently occurs every two or three 

 weeks, but its existence is not very evident, except in the pres- 

 ence of the male. In carnivora ovulation ordinarily occurs semi- 

 annually in late winter and early autumn. 



