40 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The mammary or milk glands are now known to be modified 

 tubular glands,, possibly derived from sweat glands. In the mono- 

 tremes the simplest condition is found, numbers of glands opening 

 into a pair of sacs in the sides of the marsupium, or pouch where the 

 young are kept, on the ventral side of the body (fig. 32, ^). In 

 the marsupials there is a slight nipple developed from the bottom of 

 the pocket. In the higher groups of mammals the first appearance 

 of the milk glands is the formation of a 'milk line,' a ridge on either 

 side of the body from in front back to the inguinal region. This is 

 soon divided into 'milk points,' from each of which there is an 

 ingrowth of epidermis into the corium, the intermediate parts of the 



Fig. 32. — Scheme of different kinds of nipples, based on figures by Weber. Single 

 line, ordinary integument, double line, that of primary mammary pocket. A, primitive 

 condition, found in Echidna; B, human nipple; D, Didelphys before lactation; C, same at 

 lactation; E, embryonic, F adult conditions in cow. B and C are true nipples, P a 

 pseudo-nipple (teat). 



line disappearing. Each of the points may develop into a definitive 

 mamma or milk gland, but not all of them come to full development; 

 for the number in the adult is less than in the embryo, varying from 

 a single pair in many mammals to eleven in Centetes, the number 

 roughly corresponding to the number of young at a birth. This 

 method of formation explains the varying position of the mammae 

 and also the occasional occurrence of more than the normal number 

 (polymastism) in man and other mammals. Each gland is provided 

 with a nipple and of these there are two kinds (fig. 32). In the one 

 the whole surface on which the lacteal ducts empty becomes elevated, 



