ATAVISM. 163 



traversed by the excretory ducts of the gland, or forms 

 a conduit whereby the young are enabled readily to suck 

 the secretion. Mammary glands without ducts occur 

 normally only in ornithorJiyndms and echidna. Milk- 

 glands are a distinguishing feature of mammalia, being 

 unknown below this class. 



Mammary glands are, as a rule, regularly and sym- 

 metrically arranged along the ventral aspect of the trunk, 

 in two rows. When numerous they extend along the 

 thorax and abdomen into the inguinal region. The teats 

 usually correspond to the maximum number of young at 

 a birth, but to this rule there are many exceptions. The 

 greatest number known is fourteen pairs. When the 

 number of functional glands is reduced to two, traces of 

 the suppressed mammae occur as supernumerary or 

 accessory nipples, with or without rudimentary glands. 



The reduction in number may take place in the 

 thoracic region, leaving the inguinal set functional as in 

 the cow, mare, sheep, goat, deer, and antelope. The 

 inguinal glands may abort, leaving the pectoral set 

 functional as in man, dugong, sloth, manatee, and 

 monkeys. In marsupials the milk-glands are inguinal 

 in position, and protected by the remarkable fold of 

 skin which forms the pouch or marsupium. 



The lemurs offer instructive modifications. The ring- 

 tailed lemur (L. catta\ the black lemur (L. macaco], and 

 the mongoose lemur (L. mongoz\ possess one pair of 

 pectoral mammae, as in man and quadrumana. Co- 

 querel's mouse lemur (Chirogaleus coquereli\ and the 

 dwarf lemur (Microcebus smithi), possess three pair of 

 mammary glands : one pair in the pectoral, another in 



