366 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



first trace of the hair appears close to this papilla, but 

 soon increases in length, and when the end of the hair 

 projects from the surface, the original solid process of 

 the epidermis becomes converted into an open pit, the 

 lumen of which is filled by the root of the hair. 



The development of nails has been already described 

 on p. 283. 



Glands. The secretory part of the various glandular 

 structures belonging to the skin is invariably formed 

 from the epidermis. In Mammalia it appears that 

 these glands are always formed as solid ingrowths of the 

 Malpighian layer. The ends of these ingrowths dilate 

 to form the true glandular part of the organs, while the 

 stalks connecting the glandular portions with the sur- 

 face form the ducts. In the case of the sweat-glands 

 the lumen of the duct becomes first established ; its 

 formation is inaugurated by the appearance of the 

 cuticle, and appears first at the inner end of the duct 

 and thence extends outwards. In the sebaceous glands 

 the first secretion is formed by a fatty modification of 

 the whole of the central cells of the gland. 



The muscular layer of the secreting part of the 

 sweat-glands is said to be formed from a modification of 

 the deeper layer of the epidermic cells. 



The mammary glands arise in essentially the same 

 manner as the other glands of the skin. The glands of 

 each side are formed as a soHd bud of the Malpighian 

 layer of the epidermis. From this bud processes sprout 

 out, each of which gives rise to one of the numerous 

 glands of which the whole organ is formed. 



