horny outer layei 



bone 



.epidermis 



shed 



-hairs in horn 



GIRAFFE 



abscission line 



"hair" 



Tui-tjM-riy ^ L^ 



A RUPICAPRA B ANTILOCAPRA D RHINOCEROS F CERVUS 



Figure 8-5. Horns of different types; their growth and replacement. A, chomois or cow (continuous 

 growth); B, Prong-horned Antilope (shed annually); C, giraffe with knobs of skull covered by skin 

 (continuous growth); D, fibrous or "hair" horn of rhinoceros (continuous growth); E, detail of hairs 

 embedded in matrix of rhinoceros horn; F, bony horn of the deer (shed annually). (After Erdmann, 

 1934) 



licles are involved in the origin of the mammary glands, 

 which suggests that they are modified sebaceous glands. 



The areolar glands of Montgomery are about interme- 

 diate in form between the apocrine sweat gland and the 

 mammary glands, thus indicating the possibility of transition 

 from sweat to mammary gland. The apocrine sweat glands 

 and sebaceous glands at the edge of the areola of the nipple 

 open together. These glands may have developed from lacta- 

 tion hair follicles. The intergradation of sweat, mammary, 

 and sebaceous glands suggests they are all intimately related. 



In monotremes, the mammary glands originate from hair 

 follicles that form two bands along the belly of the embryo; 

 these bands are called the milk lines. Mammary develop)- 

 ment is limited to bilateral areas of these strips on the lower 

 part of the belly, and nipples are not formed. The glands 



open on a pair of bare areas in Ornithorhynchus, and, 

 in the Echidna, into a pair of invaginated pockets lying in 

 the side walls of the marsupium, or pouch. 



In marsupials, several mammary glands arise from paired 

 invaginations of epidermis along the milk lines on the lower 

 belly. From the bottom of these invaginations, hair follicles 

 develop, and from these, mammary and sebaceous glands 

 arise. Later the hair follicles degenerate, leaving the glands 

 opening into the bottom of the nipple primordeum, which 

 then evaginates. The several pairs of nipples are located in 

 the marsupium. 



In the mammal, the nipple primordea develop along the 

 milk ridge as in the marsupial. These anlagen are associated 

 with mammary hairs which, if they persisted, would project 

 from the tip of the nipple — through the openings of the 



THE SKIN AND ITS DERIVATIVES 



