DEVELOPMENT OF THE SKIN 587 



5) Development of the Skin Glands. Three types of glands develop in re- 

 lation to the skin in mammals: 



( 1 ) sebaceous or oil glands, 



(2) sudoriferous or sweat glands, and 



(3) mammary or milk glands. 



a) Sebaceous Glands. Sebaceous glands generally are associated with 

 the hair follicles (figs. 272G; 273A), but in some areas of the body this 

 association may not occur. For example, in the human, sebaceous glands 

 arise independently as invaginations of the epidermis in the region of the 

 upper eyelids, around the nostrils, on the external genitals, and around the 

 anus. When the sebaceous gland arises with the hair follicle, it generally takes 

 its origin from the lower side of the invaginated hair follicle, although this 

 condition may vary (fig. 272G). The sebaceous-gland rudiment originates as 

 an outpushing of the germinative stratum and differentiates into a simple or 

 compound alveolar type of gland. The secretion originates as fatty material 

 within the more centrally located cells of the gland, with subsequent degen- 

 eration of these cells and release of the oily substance. Since the secretion 

 forms as a result of alteration of the gland cells themselves, this type of gland 

 is classified as an holocrine gland. New cells are formed continuously from 

 that portion of the gland connected with the germinative stratum. The oil 

 produced is discharged to the surface of the skin through the opening of the 

 hair follicle when a relationship with the hair is present. If not connected with 

 a hair follicle, the gland has a separate opening through the epidermal layer. 



b) Sudoriferous Glands. Sweat or sudoriferous glands most often de- 

 velop independently of hair follicles, but in certain areas they form on the 

 sides of these follicles. Whenever formed, they represent solid, elongated in- 

 growths of the epidermis into the dermis. Later these cellular cords coil at 

 their distal ends to form simple, coiled, tubular glands (fig. 276). 



The outer wall of the forming sweat gland develops so-called myoepithelial 

 cells; the latter presumably have the ability to contract. The cells lining the 

 lumen of the gland secrete (excrete) the sweat, the distal ends of the cells 

 being discharged with the exudate. Hence, this type of gland is called an 

 apocrine gland. The secretion is watery and contains salts, wastes, including 

 urea, and occasionally some pigment granules and fat droplets. In the cat, dog, 

 and other carnivores, sweat glands are reduced in number. 



c) Mammary Glands. Mammary glands are characteristic of the mam- 

 mals. The first indication of mammary-gland development is the formation of 

 the milk or mammary ridges (fig. 24 ID, E). These ridges represent elevations 

 of the epidermis, extending along the ventro-lateral aspect of the embryo 

 from the pectoral area posteriad into the inguinal region. The ridges are de- 

 veloped in both sexes and represent a generalized condition of development. 

 In the human embryo, the mammary ridge is well developed only in the 



