204 THE EVOLUTION OF MAN. 



outer-skin (epidermis), which extends into the leather-skin 

 (coriwm), and there branches. In the new-bom child it 

 consists merely of from twelve to eighteen radiating lobules 

 (Fig. 216). These gradually branch, the excretory passages 

 become hollow, and a large quantity of fatty matter collects 

 between the lobules. Thus is developed the prominent 

 breast of the female (mamma), on the summit of which 

 rises the nipple (onam.m.illa), adapted for being sucked.^®^ 

 The nipple does not appear until after the milk-gland 

 is already formed; this ontogenetic phenomenon is very 

 interesting, because the more ancient Mammals (the parent- 

 forms of the entire class) had no nipples. In them, the milk 

 simply emerged through a plane, sieve-like perforated spot 

 in the abdominal skin, as is even now the case in the lowest 

 extant Mammals, the Beaked Animals (Monotremata ; p. 146). 

 On account of this character these animals may be called 

 Am/ista (without nipple). In many of the lower mammals 

 there are numerous milk-glands, situated at various points of 

 the ventral side. In the human female there is usually only 

 a pair of milk-glands, placed on the point of the breast, as in 

 A.pes, Bats, Elephants, and some other Mammals. Occasion- 

 ally, however, even in the human female two pairs of breast 

 glands (or even more) appear, lying one behind the other ; 

 this must be regarded as a reversion to an older parent- 

 form. Sometimes these glands are well developed even in 

 the male, and are capable of being sucked, though as a rule 

 they exist in the male sex only as rudimentary organs with- 

 out function. 



Just as the skin glands originate as local growths of 

 the outer skin in an inward direction, so the appendages 

 of the skin, called hair and nails, originate as local growths 



