580 Comparative Morphology of Chordates 



functional only during the period of rearing young. In males, they are 

 present in a rudimentary state. 



The wide range of difference in number and position of the mam- 

 mary organs in the several groups of mammals is, in some degree, 

 made intelligible by the common occurrence of the milk-lines in mam- 

 malian embryos. The milk-line apparently is potentially mammary 

 throughout its entire length. In a given species it produces organs 

 whose number and position are appropriate in relation to the number 



Fig. 449. The occasional presence of 

 supernumerary teats (polymastism) in man 

 indicates the genetic continuity of man and 

 other mammals. (After Wiedersheim. Cour- 

 tesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



of young and the conditions under which suckling takes place in that 

 species. Within a species, especially one which normally has a large 

 number of mammary organs, there is commonly some variation in the 

 number. Sometimes, even in man, and from unknown causes, the milk- 

 lines produce organs in excess of the normal number and in unusual 

 localities. In the human female the "supernumerary" organs, situated 

 usually above the normal pair, may become functional (polymas- 

 tism). In human males supernumerary nipples are not uncommon 

 (polythelism), even to such extreme overproduction as that of the 

 case represented in Fig. 449. 



In its highly glandular character mammalian skin resembles am- 

 phibian rather than reptilian skin. It is possible that the very early 



