THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 



413 



having disappeared. Later the central cells of the epidermal mass become 

 cornified and are cast off, leaving a depression in the skin (Fig. 356). In em- 

 bryos of 250 mm. a number of solid secondary buds have grown out (Fig. 357). 

 These resemble the anlagen of the sweat glands, to which they are generally 

 considered as closely allied (Hertwig, Wiedersheim and others), and represent 

 the excretory ducts. Continued evaginations from the terminal parts of the 

 excretory ducts form the lobular ducts and acini. The acini, however, are 

 scarcely demonstrable in the male, and not even in the female until pregnancy. 

 Lumina appear by a separation and breaking down of the central cells of the 

 ducts and acini, the peripheral cells remaining as their lining. 



Epitrichium 



Nipple 

 depression 



Dermis 



Stratum 



Stratum 

 germinativum 



Dermis 

 (Areolar zone) 



FIG. 356. Vertical section through the anlage of the mammary gland of 

 a human foetus of 16 cm. Bonnet. 



Late in fretal life, or sometimes after birth, the original depressed gland 

 area becomes elevated above the surface to form the nipple. The excretory 

 ducts (15 to 20 in number) which at first opened into the depression, thus come 

 to open on the surface of the nipple. In the area around the nipple the 

 areola numerous sudoriferous and sebaceous glands develop, some of which 

 come to open into the lacteal ducts. Sometimes rudimentary hairs appear. 

 Other glands known as areolar glands (of Montgomery) resembling rudi- 

 mentary mammary glands also develop from the epidermis of the areola. 



After birth the mammary glands continue to grow slowly in both sexes up to 

 the time of puberty. After this they cease to grow in the male, and then atrophy. 

 In the female, growth of the glandular elements goes on, but very slowly, and 

 usually a considerable amount of fat develops in the surrounding tissue, 

 causing the enlargement of the breasts. 



The Mammary Glands of Pregnancy. Even in the female, as stated before, 

 acini are scarcely demonstrable until pregnancy. The mamma consists 



