THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 449 



formed hairs growing up through the old sheaths. In some cases, however, new 

 follicles are formed directly from the epidermis and dermis. In some of the 

 lower Mammals, new hair germs appear as outgrows from the sheaths of old 

 follicles, thus giving rise to tufts of hair. The arrectores pilorum muscles arise 

 from the dermal (mesenchymal) cells and become attached to the follicles below 

 the sebaceous glands. 



The Glands of the Skin. 



The Sebaceous Glands. — These structures usually develop in connection 

 with hairs. From the root sheath a solid bud of cells grows out into the dermis 

 (Fig. 393, IV) and becomes lobed. The central cells of the mass undergo fatty 

 degeneration and the products of degeneration pass to the surface of the skin 

 through the space between the hair and its root sheath. The more peripheral 

 cells proliferate and give rise to new central cells which in turn are transformed 

 into the specific secretion of the gland, the whole process being continuous. On 

 the margins of the lips, on the labia minora and on the glans penis and prepuce, 

 glands similar in character to the sebaceous glands arise directly from the 

 epidermis independently of hairs. 



The Sudoriferous Glands. — The sweat glands begin to develop during 

 the fifth month as solid cylindrical growths from the deeper layers of the epider- 

 mis into the dermis (Fig. 391). Later the deeper ends of the cylinders become 

 coiled and lumina appear. The lumina do not at first open upon the surface 

 but gradually approach it as the deeper epidermal layers replace the more 

 superficial. 



The Vernix Caseosa. — During fcetal life the secretion of the sebaceous 

 glands becomes mingled with the cast-off epitrichial and epidermal cells to form 

 the whitish oleaginous substance (sometimes called the smegma embryonum) 

 that covers the skin of the new-born child. It is collected especially in the 

 axilla, groin and folds of the neck. 



The Mammary Glands. 



In embryos of six to seven mm., or even less, a thickening of the epidermis 

 occurs in a narrow zone along the ventro-lateral surface of the body (Strahl). 

 In embryos of 15 mm. this thickening, known as the milk ridge, extends from the 

 upper extremity to the inguinal region (Kallius, Schmidt). Later the caudal 

 end of the ridge disappears, while the cephalic portion becomes more prominent. 

 The further history of the ridge has not been traced, but in embryos considerably 

 older the anlage of each gland is a circular thickening of the epidermis in the 

 thoracic region, projecting into the underlying dermis. It seems most probable 

 that this local thickening represents a portion of the original ridge, the remainder 



