136 THE PHYSIOLOGY OF REPRODUCTION 



In the ferret ovulation occurs during oestrus, but postpone- 

 ment of coition may bring about the degeneration of the ripe 

 follicles, since they do not always discharge spontaneously. 1 



Artificial insemination, followed by pregnancy, has been 

 successfully performed on mares, donkeys, and cows. 2 Conse- 

 quently it may be concluded that these animals ovulate inde- 

 pendently of coition. According to Ewart, 3 ovulation in the 

 mare very often does not occur until near the end of the oestrous 

 period. 



It has been shown also that the sheep ovulates spontaneously 

 at each of the earlier heat periods of the sexual season, but that 

 there are reasons for believing that during the later periods 

 the stimulating power at the disposal of the ewes may be so 

 reduced that without coition it is incapable of causing ovulation. 

 There is also evidence that when coition occurs at the beginning 

 of an oestrous period, it may provide a stimulus inducing ovula- 

 tion to take place a few hours earlier than it otherwise would ; 

 in other words, that if ovulation has not already occurred during 

 an oestrus, the stimulus set up by coition may hasten the rupture 

 of the follicle. 4 Recently IwanofT has succeeded in inducing 

 pregnancy in sheep by artificial insemination. (See p. 183.) 



There can be little doubt that in the great majority of 

 Mammals ovulation, as a general rule, occurs regularly during 

 oestrus. In certain bats, however, copulation is performed 

 during the autumn, whereas ovulation is postponed until the 

 following spring, the animals in the meantime hibernating, 

 while the spermatozoa are stored up in the uterus (see p. 177). 5 

 The ovary in the winter months (during the hibernating period) 

 is said to be in a state of quiescence, and the exact time for 



1 Marshall, "The (Estrous Cycle in the Common Ferret," Quar. Jour. 

 Micr. Sci., vol. xlviii., 1904. 



2 Heape, " The Artificial Insemination of Mammals," Proc. Hoy. Soc., 

 vol. Ixi., 1897. 



3 Ewart, " The Development of the Horse," MS. 



4 Marshall, " The (Estrous Cycle and the Formation of the Corpus Luteum 

 in the Sheep," Phil. Trans., B., vol. cxcvi., 1903. 



5 Benecke, " Ueber Reifung und Befruchtung des Eies bei den Fleder- 

 mausen," Zool. Anz., vol. ii., 1879. Eimer, "Ueber die Fortpflanzung der 

 Fledermause," Zool. Anz., vol. ii., 1879. Van Beneden and Julin, " Observa- 

 tions sur la Maturation, la Ferondation, et la Segmentation de I'CEuf chez les 

 Cheiropteres," Arch, de BioL> vol. i., 1880. 



