Chap. 18 REPRODUCTION 355 



limited and relatively very short periods. In contrast to this, as already pointed 

 out, in chimpanzees, the higher apes, and man mating may occur at any time. 

 The principal means of preventing fertilization are the mechanical and 

 chemical ones that keep the sperm and egg from meeting, and the "rhythm 

 method," a restriction of the time of sexual intercourse to the periods when 

 no egg is apt to be present, i.e., avoiding those near ovulation. Authorities 

 generally agree that ovulation occurs about 14 days after the first day of men- 

 struation in a 28-day cycle, when these two functions are completely regular 

 and standardized in time. Even so, two days before and two days after the 

 supposed ovulation date are usually included in the possible ovulation time. 

 Two facts, however, must be taken into consideration. Many exceptions and 

 irregularities occur in the menstrual schedule even in the same individual. 

 Ovulation is an unobtrusive physiological process of which few persons are 

 certainly aware. Completely regular and standardized cycles are rare indeed. 



