DEVELOPMENT OF THE SKIN 579 



species, with the exception of the Cetacea (whales) and the Sirenia (sea cows) . 

 Various types of horny structures are associated with the epidermis, while 

 the dermis may develop plates of bone in certain instances. Both epidermis 

 and dermis are of considerable thickness. 



b. Development of the Skin 



1) Development of the Skin in General. As in other vertebrates, the primi- 

 tive mammalian integument is formed by the epidermal tube which, when 

 first developed, consists of a single layer, one cell in thickness (fig. 272A). 

 Later it becomes double layered, having an external flattened periderm and 

 an inner stratum germinativum. As in other vertebrates, the germinative 

 stratum is the reproductive layer. Mesenchyme condenses below the germina- 

 tive stratum, and the rudiment of the future dermis is formed (fig. 272B). 



In the further development of the epidermal layer, a third layer of cells, 

 the stratum intermedium, appears between the periderm and the stratum ger- 

 minativum (fig. 272C). The stratum germinativum or deep layer of Malpighi 

 may appear to be several cells in thickness as development proceeds. The 

 cells of the germinative stratum, in contact with the dermal surface, are 

 cuboidal or cylindrical (fig. 272C, D). During later developrpent, the epi- 

 dermis becomes highly stratified, and the outer or external layer is converted 

 into a cornified layer, the stratum corneum (fig. 272D). Cornification oc- 

 curs first on the future contact surfaces of the appendages, such as the volar 

 surface of the hand, plantar surface of the foot, and foot pads of the cat, dog, 

 etc. Pigment granules (melanin) appear in the deepest layers of the epidermis 

 in the region of the basal, cylindrical cells of the stratum germinativum during 

 later fetal development and after parturition (birth). 



In the meantime, the dermal mesenchyme increases in thickness, and vari- 

 ous types of connective-tissue fibers, white and elastic (see Chap. 15), appear 

 in the intercellular substance between the mesenchymal cells. Pigment cells 

 make their appearance in the dermis during later fetal development. These 

 cells descend, probably, from cells of neural crest origin, although other 

 mesenchymal cells possibly may contribute to the store of pigment-forming 

 cells. Fat cells occur in the deeper layers of the dermis. 



2) Development of Accessory Structures Associated with the Skin: a) 

 Development of the Hair. The first indication of hair development is the 

 formation of a localized thickening and invagination of the epidermal layer, 

 particularly the germinative stratum (fig. 272E). This thickened mass of epi- 

 dermal cells pushes inward, accompanied by an increase in the number of 

 epidermal cells in the area of invagination (fig. 272F). Adjacent mesen- 

 chymal cells of the dermis respond to this epidermal activity by aggregating 

 about the invaginating mass (fig. 272E, F). As the germinative stratum with 

 its central core of cells continues to push downward in tangential fashion 



