402 THE ORGANS 



an outer, the future stratum corneum, and an inner, the future stratum germ- 

 inativum. The stratum granulosum and stratum lucidum are special develop- 

 ments of the stratum germinativum. The corium is of mesoblastic origin. It 

 is at first smooth, the papillae being a secondary development. 



The nail first appears as a thickening of the stratum lucidum. This spreads 

 until the future nail bed is completely covered. During development the stra- 

 tum corneum extends completely over the nail as its eponychium. During the 

 ninth month (intra-uterine) the nail begins to grow forward free from its bed 

 and the eponychium disappears, except as already noted. 



The hair also develops from ectoderm. It first appears about the end of the 

 third foetal month as a small local thickening of the epidermis. This thicken- 

 ing is due mainly to proliferation of the cells of the stratum mucosum, and soon 

 pushes its way down into the underlying corium, forming a long slender cord of 

 cells — the hair germ. Differentiation of the surrounding connective tissue 

 of the corium forms the follicle wall, while an invagination of this connective 

 tissue into the lower end of the hair germ forms the papilla. The cells of the 

 hair germ now dift'erentiate into two layers: a central core the middle portion 

 of which forms the hair, while the peripheral portion forms the inner root sheath; 

 and an outer layer which becomes the outer root sheath. The sublayers are 

 formed from these by subsequent differentiation. The hair when first formed 

 lies wholly beneath the surface of the skin. As the hair reaches the surface 

 its pointed extremity pierces the surface epithelium to become the hair shaft 

 (Fig. 283). 



The sebaceous gland develops as an outgrowth from the outer root sheath. 

 This is a flask-shaped and at first solid mass of cells, which later differentiate to 

 form the ducts and alveoli of the gland. 



The sweat glands first appear as solid ingrowths of the stratum germina- 

 tivum into the underlying corium. The lower end of the ingrowth becomes 

 thickened and convoluted to form the coiled portion of the gland, and somewhat 

 later the central portion becomes channelled out to form the lumen. The 

 muscle tissue of the sweat glands, which lies between the epithelium and the 

 basement membrane, is the only muscle of the body derived from the ectoderm. 



The Mammary Gland 



The mammary gland is a compound alveolar gland. It consists 

 of from fifteen to twenty lobes, each of which is subdivided into 

 lobules. The gland is surrounded by a layer of connective tissue 

 containing more or less fat. From this periglandular connective 

 tissue broad septa extend into the gland, separating the lobes (inter- 

 lobar septa). From the latter finer connective-tissue bands pass in 

 between the lobules (interlobular septa). From the interlobular 

 septa strands of connective tissue extend into the lobule where they 

 act as support for the glandular structures proper. An excretory duct 

 passes to each lobe where it divides into a number of smaller ducts 



