CYCLES OF REPRODUCTION 17 



Other mammals in exhibiting typical oestrous cycles. 

 This is the case for instance in Tarsius, which has a 

 cycle length of about 23 days,^ and in the platyrhine 

 monkeys. The catarrhine or old-world monkeys are 

 similar except that they also show a periodic bleeding 

 from the uterus caused by a breakdown of the uterine 

 lining.203 This lining becomes thickened to receive the 

 embryo should fertilization follow ovulation, and in the 

 absence of fertilization it degenerates. All mammals 

 with oestrous cycles show a similar degeneration 

 between successive ovulations, but in the non-primate 

 the process is not usually so drastic and does not involve 

 loss of blood. Thus the difference shown by the catarr- 

 hine cycle is one of degree only. It is unfortunate that 

 this cycle is so often regarded as being something unique, 

 and that it has been distinguished by the special name of 

 menstrual cycle. Indeed the name is doubly unfortunate 

 since not all menstrual cycles have an average periodicity 

 of one month. In Azara's capuchin the cycle length is 

 about 16 days, and in the gorilla about 45 days.® 



The human cycle has an average length of about 28 

 days, with 97 per cent of cycles lying between 22 and 

 36 days,ioo and this, of course, is the original menstrual 

 cycle. Within this cycle it is generally agreed that the 

 commonest time for ovulation is at about the middle of 

 the interval between successive menstrual periods, the 

 times when bleeding occurs. However, individual 

 variations from this rule are common, and in extreme 

 cases ovulation may even occur during menstruation. ^^ 

 It is to be regretted that no detailed study of this cycle 

 has yet been made. 



In conclusion an exception must be mentioned to 

 the general rule that female mammals with extended 

 breeding seasons experience some form of oestrous 

 cycle with periodic and spontaneous shedding of 

 the eggs. In a small number of species, of which the 

 rabbit is the best known, the non-pregnant animal is 

 always ready to mate, and it is the nervous stimulation 



