CHAPTER XVI. 



THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 



The integument consists of the skin and certain accessory structures. The 

 skin is composed of the dermis (or corium) and the epidermis. The accessory- 

 structures comprise the hairs, nails, sudoriferous glands, sebaceous glands, and 

 mammary glands. The epidermis (or epithelial layer) and all the accessory 

 structures are derived from the ectoderm; the dermis is mesodermal in its 

 origin. Other appendages of the skin — such as scales, feathers, claws, hoofs, 

 and horns — which are found only in the lower animals, are ectodermal 

 derivatives and belong in the same class as the accessory structures in man. 



The Skin. 



The Epidermis. — The embryonic ectoderm consists primarily of a single 

 layer of cells (Fig. 81). During the latter part of the first month, the single 

 layer gives rise to two layers, of which the outer is composed of irregular flat 

 cells and is known as the epitrichium or periderm, the inner or basal, of larger 

 cuboidal cells which are the progenitors of the epidermal cells and of the acces- 

 sory structures. The epitrichial cells later become dome-shaped and acquire 

 a vesicular structure, the nuclei becoming less distinct. They persist until the 

 middle of fcetal life and are then cast off and mingle with the secretion of the 

 newly formed sebaceous glands as a constituent of the vernix caseosa (see p. 449) . 

 The epidermal cells, constantly increasing in number, soon come to form several 

 layers (4 to 6 in the sixth month) . The innermost layer rests upon the base- 

 ment membrane and is composed of cuboidal or columnar cells rich in cytoplasm ; 

 the outer layers consist of irregular cells with clearer contents and less distinct 

 nuclei. 



As development proceeds, the basal layer gives rise to several layers which 

 together constitute the stratum germinativum. The cells of the innermost 

 layers are constantly proliferating and thus forming new cells which are pushed 

 toward the surface. During the seventh month keratohyalin granules appear 

 in two or three layers which are then known collectively as the stratum granu- 

 losum. The clearer cells of the superficial layers undergo a process of de- 

 generation by which their contents are transformed into a horny substance, 

 the nuclei becoming fainter and finally disappearing. These modified or degen- 

 erated cells, which are constantly being cast off and replaced by others from 



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