SWEAT GLANDS MAMMARY GLANDS 307 



bulb to the level of the sebaceous glands, where it disappears. The hair grows 

 at the base and is pushed out through the central cavity of the anlage, the cells 

 of which degenerate. When the hair projects above the surface of the epidermis 

 it carries with it, and breaks up, the epitrichial layer. The mesenchymal tissue 

 which surrounds the hair follicle in the neighborhood of the epithelial bed gives 

 rise to the smooth fibers of the arrector pili muscles. Pigment granules develop 

 in the basal cells of the hair and give it its characteristic color. 



The first generation of hairs are short-lived and begin to degenerate before 

 birth ; usually the hair of the head is shed during the first and second years after 

 birth, and new hairs develop as buds from the old hair follicles. 



SWEAT GLANDS 



The sweat or sudoriparous glands begin to develop in the fourth month from 

 the epidermis of the finger-tips, of the palms of the hands and soles of the feet : 

 They are formed as solid downgrowths from the epidermis, but differ from hair 

 anlages in having no mesenchymal papillae at their bases. During the sixth 

 month the tubular anlages of the gland begin to coil and in the seventh month 

 their lumina appear. The inner layer of cells forms the gland cells while the outer 

 cells become transformed into smooth muscle fibers which here arise from the 

 ectoderm. In the axillary region sweat glands occur which are large and branched. 



MAMMARY GLANDS 



The tubular mammary glands peculiar to mammals are regarded as modified 

 sweat glands. In early embryos an ectodermal thickening extends ventro- 

 laterally between the bases of the limb buds on either side. This linear 

 epidermal thickening is the milk line. In the future pectoral region of this line 

 by the thickening and downgrowth of the epidermis there is formed the papilla- 

 like anlage of the mammary gland (Fig. 295 A). From this epithelial anlage buds 

 appear (B) which elongate and form solid cords 15 to 20 in number, the anlages 

 of the milk ducts (Fig. 295 C). These branch in the mesenchymal tissue of the 

 corium and eventually produce the alveolar end-pieces of the mammary glands. 

 In the region where the milk ducts open on the surface the epidermis is evagi- 

 nated to form the nipple. The glands enlarge at birth, at puberty and after par- 

 turition when they become functionally active. 



The mammary glands are homologised with sweat glands because their development is 

 similar, and because in the lower mammals their structure is the same. In many mammals 



