584 THE PHYSIOLOGY OF REPRODUCTION 



Tegetmeier and Sutherland l that mules may yield milk in sufficient 

 abundance to rear a foal. He concludes, therefore, that the source 

 of the stimulus which excites the development of the mammary 

 glands is to be sought in the ovary rather than in the foetus. 



Instances have also been recorded by Knott 2 and others, 

 in which males have secreted milk, thus showing that mammary 

 development is not necessarily even a female function ; but 

 such cases are at all events exceedingly rare. 3 Knott mentions 

 cases in which suckling occurred in a bull, a male goat, a wether, 

 and in men. He also cites instances of virgin girls who were 

 nurses secreting copious supplies of milk as a consequence of 

 allowing infants to suck their nipples ; and thus he supports 

 Heape's objection to the foetal hormone theory. Gellhorn 4 cites 

 similar cases, including one of a virgin monkey (Cercopithecus). 

 Another case is mentioned of a woman who suckled children un- 

 interruptedly for forty-seven years, and in her eighty-first year 

 had a moderate but regular supply of milk, 5 thus showing that 

 mammary secretion may continue exceptionally for long after the 

 menopause, and presumably, therefore, in the absence of any sort 

 of stimulus from the generative organs. This observation further 

 supports the idea referred to above, that normal suckling acts 

 by itself as a physiological stimulus for mammary secretion. 



A more forcible objection to the theory of the foetal hormone 

 is supplied by the Monotremata, which are the lowest order of 

 Mammalia. These animals are oviparous, the developing 

 embryo being contained in an egg, which does not enter into 

 any sort of connection with the uterine wall. Halban, 6 however, 

 has made the suggestion, which is quoted by Miss Lane-Claypon 

 and Starling, that since the embryo goes on increasing in size 

 during its passage down the female generative tract, and since the 

 shell of the egg is porous, it is not impossible that substances may 

 diffuse outward from the embryo and be absorbed by the uterine 

 mucous membrane, and so be carried into the maternal circulation. 



1 Tegetmeier and Sutherland, Horses, Asses, Zebras, Mules, and Mule 

 Breeding, London, 1895. 



* Knott, "Abnormal Lactation," &c., American Medicine, vol. ii. (new 

 series, June), 1907. Cf. Wiedersheim (see p. 555). 



3 The occasional occurrence of milk secretion in the newly born, both 

 males and females, is well known. 



4 Gellhorn, loc. cit. 5 Knott, loc. cit. Halban, loc. cit. 



