94 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION 



of the ovarian hormones, progesterone and estrogen. Why certain mammals 

 should experience violent endometrial changes evident in menstruation and 

 others a gradual involution and resorption is a question for the future. The 

 generaPperiod of change following estrus in a non-fertile cycle is known as 

 metestrus (fig. 53). In the rat and mouse, metestrus is short, about one or 

 two days; in the human and opossum it occupies approximately ten days to 

 two weeks of the cycle; in the dog, about 40 to 50 days, depending upon 

 the pseudopregnant conditions experienced in different females. The word 

 anestrus is applied to a prolonged diestrus or sexual quiescence between two 

 sexual cycles. However, the involution experienced by the sexual organs in 

 anestrus is somewhat more profound than that prevailing during a brief 

 diestrus. The term lactational diestrus is used to refer to the prolonged dies- 

 trous condition in forms such as the rat, wherein estrus is suppressed in the 

 mother while suckling the young. 



The length of the sexual cycle varies with the species. When females of 

 the rat or mouse are kept away from a male, the estrous or sexual cycle will 

 repeat itself every 4 to 5 days. In the sow it occurs every 17 to 20 days. In 

 the opossum there is a prolonged anestrous period during the summer and 

 autumn months followed by a polyestrous period during the winter and spring 

 when the estrous cycle reoccurs about every 28 days. In the human female, 

 the sexual cycle occupies about 28 days, and there are probably about ten 

 normal ovulatory cycles in a year. Some human females may have more, 

 while others experience a slightly smaller number of true ovulatory cycles 

 per year. 



Many mammals have one estrous cycle per year. This condition, known as 

 monestrus, is true of most wild mammals, such as the deer, wolf, fox, moose, 

 and coyote. In the shrew, mink, and ferret the monestrous period may be 

 prolonged if the female is kept away from the male. 



Various types of polyestrous conditions exist. In the female dog, for ex- 

 ample, there are two or three estrous periods per year about 4 to 6 months 

 apart. In the cat there are several cycles about two weeks apart during the 

 autumn, winter, and spring. In the domestic sheep there is a polyestrous period 

 from September to February in which the cycles occur about every 17 days, 

 followed by an anestrous period from early March to September. In the mare 

 in North America, estrous cycles of about 19 to 23 days occur from March 

 to August. In South America the breeding season is reversed, corresponding 

 to the reversed seasonal conditions south of the equator. In England many 

 mares breed in autumn and winter (Asdell, '46). 



In some mammals estrus may follow immediately after parturition or birth 

 of the young. This may occur occasionally in the rat. Under normal conditions 

 in the fur seal, the female lactates and gestates simultaneously. It is not a 

 common procedure. 



It should be observed that there are two aspects of the female reproductive 



