MATURATION. 25 



observations, but they show that in general ovulation occurs during the 

 oestrus or period of "heat" in the female, sometimes coincident with copu- 

 lation. Just prior to the oestrus period there is a marked increase of blood 

 flow to the generative organs, during a pro-cestrual period or pro-cestrus. 

 During oestrus the increased blood flow is maintained and may be accen- 

 tuated at the approach of the male, and it has been suggested that an in- 

 crease in blood pressure in the ovary is at least one of the factors in causing 

 the rupture of the Graafian follicle. Another contributing factor may be 

 an increase in the quantity of fluid within the follicle thereby increasing 

 the intrafollicular pressure. 



In monkeys there is a slight menstrual flow which may occur periodically 

 the year round, but there seems to be a limited season for ovulation and con- 

 ception. Menstruation and ovulation therefore do not necessarily coincide. 

 In the human the menstrual flow is a pronounced feature during the years of 

 reproductive activity of the female, recurring at average intervals of 28 days 

 except during pregnancy and usually during lactation. It is generally ad- 

 mitted that the time of menstrual flow corresponds to the pro-cestrual period 

 of the lower mammals, that is, the period immediately preceding the oestrus 

 or rutting time. It would be expected that in the human female the period 

 of sexual desire would follow menstruation. It seems, however, that condi- 

 tions of modern society have disturbed the natural cycle of physiological 

 activities, although there is reason to believe that in primitive man there was 

 at least an approximation to conditions in the lower mammals. In highly 

 civilized man there appears to be no particular period of sexual desire, and 

 there is considerable evidence that ovulatiqn is not always associated with 

 menstruation but may occur at any time during the intermenstrual period. 

 With the disappearance of a fixed oestrus in the human female the definite 

 relation between ovulation and the oestrus has broken down, although bio- 

 logically the most favorable condition for conception is ensemination just 

 after the menstrual flow. 



Earlier in this chapter it was stated that the number of ova in the two 

 ovaries approximated 70,000. Allowing one ovum to each ovulation, not 

 more than about 400 of these attain maturity during the years of a woman's 

 reproductive activity, the others along with their follicles probably degener- 

 ating within the ovaries. The general concensus of opinion is that in the great 

 majority of cases only one ovum escapes at ovulation either from one ovary 

 or the other. One possible exception to this occurs in the case of twin off- 

 spring where the twins are not identical. There is good evidence that iden- 

 tical twins arise from a single ovum, and it is not impossible even that ordinary 

 twins develop from the same ovum. 



