56 INTRODUCTION TO SEXUAL PHYSIOLOGY 



times, and conversely, periods when desire is lacking. Neverthe- 

 less, there is considerable disagreement as to the stage in the 

 cycle at which the oestrous periods occur, biit there would appear 

 good ground for stating that they are most frequent in the week 

 after menstruation (the period being sometimes extended for a 

 fortnight). It must be remembered that luxurious conditions 

 favour the continuance of oestrus, and that even in some of 

 the lower animals, when stimulated by abundant food supply 

 (e.g. domesticated rabbits), coition may often occur in early 

 pregnancy when it could serve no purpose in perpetuating the race. 



In the monkey {Macacus) Corner has shown that ovulation 

 takes place twelve to fourteen days before the commencement of 

 menstruation or about ten days after its cessation (the flow 

 lasting from four to six days). The time of ovulation may 

 therefore correspond to a point in the cycle at or near the termina- 

 tion of a natural oestrus (if there ever be one), supposing oestrus 

 to succeed the prooestrum as in the lower mammals. For man, 

 such evidence as is available points rather in the same direction. 

 Siegel, taking advantage of the fact that during the Great War 

 married soldiers in Germany only had occasional leave from the 

 army and then only for a very few days at a time, was able to 

 obtain data as to the stage in the cycle at which women were 

 most liable to conceive. He found that the probability of a 

 union being fertile increased from the beginning of the menstrual 

 discharge until six days subsequently when it reached its 

 maximum ; it remained at approximately the same height until 

 about the thirteenth day, and then declined until the twenty- 

 second day after the commencement of the flow ; while from the 

 twenty-second to the twenty-eighth day the unions were completely 

 sterile. This evidence then points to the conclusion that ovulation 

 is most frequent from the sixth to the thirteenth day (or possibly 

 up to the tenth, since it is improbable that the discharged ova 

 die within three or four days) after the beginning of menstruation. 

 This time would correspond more or less to a post-menstrual 

 oestrous period. Moreover Mall, as a result of a study of thirty- 

 six cases of early human embryos, has come to the conclusion 

 that fertile coition is most likely to occur between the fourth 

 anp thirteenth day after tlie first appearance of the menstrual 

 discharge. 



It has been suggested that whereas monkeys apparently 



