In many of the lower mammals we find a number of milk-glands at different parts of the ventral surface. In the human female there is usually only one pair of glands, at the breast; and it is the same with the apes, bats, elephants, and several other mammals. Sometimes, however, we find two successive pairs of glands (or even more) in the human female. Some women have four or five pairs of breasts, like pigs and hedgehogs (Figure 1.103). This polymastism points back to an older stem-form. We often find these accessory breasts in the male also (Figure 1.103 D). Sometimes, moreover, the normal mammary glands are fully developed and can suckle in the male; but as a rule they are merely rudimentary organs without functions in the male. We have already (Chapter 1.11) dealt with this remarkable and interesting instance of atavism. (FIGURE 2.287. The female breast (mamma) in vertical section. c racemose glandular lobes, b enlarged milk-ducts, a narrower outlets, which open into the nipple. (From H. Meyer.)) While the cutaneous glands are inner growths of the epidermis, the appendages which we call hairs and nails are external local growths in it. The nails (Ungues) which form important protective structures on the back of the most sensitive parts of our limbs, the tips of the fingers and toes, are horny growths of the epidermis, which we share with the apes. The lower mammals usually have claws instead of them; the ungulates, hoofs. The stem-form of the mammals certainly had claws; we find them in a rudimentary form even in the salamander. The horny claws are highly developed in most of the reptiles (Figure 2.264), and the mammals have inherited them from the earliest representatives of this class, the stem-reptiles (Tocosauria). Like the hoofs (ungulae) of the Ungulates, the nails of apes and men have been evolved from the claws of the older mammals. In the human embryo the first rudiment of the nails is found (between the horny and the mucous stratum of the epidermis) in the fourth month. But their edges do not penetrate through until the end of the sixth month. The most interesting and important appendages of the epidermis are the hairs; on account of their peculiar composition and origin we must regard them as highly characteristic of the whole mammalian class. It is true that we also find hairs in many of the lower animals, such as insects and worms. But these hairs, like the hairs of plants, are thread-like appendages of the surface, and differ entirely from the hairs of the mammals in the details of their structure and development.