GESTATION AND LACTATION 81 



74. The mammary glands. All mammals are supplied 

 with milk-secreting glands. These glands are present 

 in both sexes, but are rudimentary in the male. In the 

 female the glands are large and are stimulated into active 

 functioning by the exercise of the reproductive organs. 

 In the immature female before the arrival of puberty, 

 these glands are small and inconspicuous. With the 

 first appearance of puberty accompanied by the oestrum, 

 the glands increase in size. 



The number of glands present varies with the species. 

 In some animals which normally give birth to one offspring 

 at a time, as in man, there are two glands (mammae). 

 In the cow, however, normally producing one at a birth, 

 the normal number of mammae is four. In cats, dogs, 

 rabbits and swine, species producing from four to twenty 

 young at one time, there are normally present several 

 pairs of mammae. Wentworth found the number of 

 mammae in swine to vary from nine to fourteen. 1 There 

 is, therefore, a rather definite relation between the num- 

 ber of mammae and the normal number of young pro- 

 duced at each birth. This relation does not seem to be 

 important as an index of fertility in any particular species. 



75. The duration of lactation. Among wild forms the 

 continuance of lactation varies widely in different species. 

 In general the period of lactation ends when the young 

 animal has developed to a point where it can live in- 

 dependently and secure its nourishment in the same way as 

 thejnature individual. In the case of the domestic cow, 

 the milking function has been developed and stimulated 

 under domestication to a point where the lactation period 

 may persist from one calving period to the next without 



1 Wentworth, "Inheritance of Mammae in Duroc- Jersey 

 Swine," American Naturalist, vol. 47, 1913. 



