74 THE PHYSIOLOGY OF REPRODUCTION 



formed by cutaneous folds in the vaginal region. In Mono- 

 tremes the young are hatched from eggs which, after being laid, 

 are deposited in the pouch. 



The question as to what are the precise factors which de- 

 termine the length of the gestation period has already been 

 referred to in the first chapter, where it was pointed out that both 

 the duration of pregnancy and the time of the year at which 

 breeding occurs are necessarily controlled by natural selection, 

 acting in the interests of the next generation. 



The effects of lactation upon the recurrence of oestrus vary 

 widely, and are often different among individuals belonging to 

 the same species. Thus, although the mare as a general rule 

 is capable of becoming pregnant while suckling, in some 

 individuals the sexual season is postponed, the mares only 

 becoming pregnant once in two years. 



The return of menstruation during lactation in women has 

 been dealt with recently by Heil, 1 and Ding wall Fordyce. 2 

 Heil, who had studied the conditions of two hundred nursing 

 mothers, expresses the belief that the occurrence of menstruation 

 and not the condition of amenorrhcea is the normal state 

 during lactation, but that menstruation is not so frequent in the 

 later lactations as during the earlier ones. Fordyce has reached 

 similar conclusions, finding that menstruation occurred during 

 lactation in forty per cent, of the cases in which suckling was 

 performed, while in ninety-two per cent, of the cases its return 

 was within nine months of parturition, and that menstrua- 

 tion during lactation was commoner with the earlier than with 

 the later lactations, showing that age is an important factor. 



The histological changes which occur in the internal gene- 

 rative organs of various Mammalia during the cestrous cycle 

 are described at some length in the succeeding chapters. 



1 Heil, " Laktation und Menstruation/' Monatsschr.f. Geburtsh. u. Gynak., 

 vol. xxiv., 1906. 



2 Fordyce, "An Investigation into the Complications and Disabilities of 

 prolonged Lactation." Being an extension of papers published in The Lancet, 

 Part I., 1906, The Brit. Med. Jour., Part I., 1906, and The Brit. Jour, of 

 Children's Diseases, 1906. Gellhorn ("Abnormal Mammary Secretion," 

 Jour. Amer. Mcd. Assoc., Nov. 21, 1908) mentions a case of an ape 

 (Cercopithecus) in which menstruation always disappeared during profuse 

 lactation, but reappeared as soon as the mammary secretion ceased or became 

 markedly decreased. 



