THE INTEGUMENT AND THE EXOSKELETON 117 



their primary position. By a suppression of the posterior 

 portion of this series, pectoral mammce are produced, either 

 two pairs, as in some lemurs, or a single pair, as in the majority 

 of primates, and in bats. In the aquatic Sirenia, also, the 

 mammae are pectoral. As they bear but a single young at 

 a time and nurse it by clasping it in the flippers while stand- 

 ing upright in the water, these animals, as suggested by the 

 name, are probably the real origin of the well-nigh universal 

 mermaid myth. The pectoral position is the most convenient 

 for arboreal animals like the Anthropoidea and enables them 

 to carry the offspring in one arm and leave the other free for 

 climbing. 



In many animals with a restricted number of mammae there 

 have been frequently observed cases of supernumerary nipples 

 or supernumerary mammae. These are termed respectively 

 hyperthelism and hypermastism, and are looked upon as 

 atavistic and indicative of the former development of a com- 

 plete series, of which those normally developed form a part. 

 They are often noted in the adult human subject, and the 

 anlagen of numerous pairs of nipples occur regularly in the 

 embryo. The occurrence of six-nippled sheep that have a 

 tendency to cast two young at a birth has been recently made 

 the subject of experiment with a view to perpetuating the latter 

 peculiarities, and thus form a race adapted to countries with 

 a short summer, like Canada. Whether there is a definite 

 correlation between these two characters, or whether in Man 

 there is any correspondence between hypermastism and a 

 tendency to produce twins, has never been determined. 



The occasional occurrence of mammae in unusual positions, 

 as on the thigh or the back, as has been noted in the human 

 subject, is a displacement, and not a reversion, and hence has 

 no normal morphological meaning. 



The occurrence of rudimentary nipples in the male is the 

 rule among placental mammals, but seems not to be the case in 

 monotremes and marsupials. If this be true, this is a definite 

 instance of the hereditary transmission to the male sex of 

 parts that developed first in the female, and formed for a long 



