NUMBER AND ARRANGEMENT OF NIPPLES 99 



Exirythely. But even in forms where the num- 

 ber is normally constant (Stenothely), some- 

 times accessory nipples or mammye are found. 

 It is a well-known fact that these anomalies, 

 which are usually called cases of hypertheiy 

 or hypermasty (polythely or polymasty) also 

 occur in a percentage of human beings. 

 The most detailed statistics we have on this 

 subject relate to the Japanese, and show that 

 \\ per cent, in males and about 5 per cent, 

 in females possess accessory nipples. We 

 have so -far no exact statistics relating to 

 Europeans, but in any case the percentage 

 would appear to be lower. 



All these diverse conditions in number and 

 arrangement of nipples in the Placentalia 

 received, twenty years ago, an explanation, 

 which, in its essentials, we can still accept. 

 This explanation is based upon the fundamental 

 discovery of 0. Schultze in 1892, that the 

 development of the mammary apparatus of the 

 Placentals begins with the appearance of the 

 so-called " milk-line," which in early embryos 

 is distinctly recognizable macroscopically as an 

 epidermal ridge extending on each side along 

 the lateral aspect of the trunk, from the axilla 



