DEVELOPMENT OF THE SKIN 563 



or cells which secrete a hard, bone-like substance, resembling the dentine of 

 the teeth of higher vertebrates (fig. 263D). This substance is closely related 

 to bone. The cells of the basal epidermal layer, overlying the dentine-like sub- 

 stance, then form an enamel organ, composed of columnar ameloblasts which 

 produce a hard, enamel-like coating over the outer portion of the conical mass 

 of dentine (fig. 263D). As this scale or "tooth-like" structure increases in 

 size, it gradually pushes the epidermis aside and projects above the surface 

 as a placoid scale (fig. 263E). Some are small, while others are large and 

 spine-like. Many different shapes and sizes of scales are formed in different 

 areas of the body (Sayles and Hershkowitz, '37). 



As the epidermis increases in thickness, unicellular glands appear within 

 the epidermal layer (fig. 263D). These glands discharge their secretion of 

 mucoid material externally, producing a slimy coating over the surface of the 

 skin. Multicellular glands appear at the bases of the spines which develop at 

 the anterior margins of the dorsal fins and in the epidermis overlying the 

 claspers of the pelvic fins of the male. 



c. Development of the Skin in the Bony Ganoid Fish, Lepisosteus 

 (Lepidosteus) osseus 



The development of the epidermis and dermis in Lepisosteus is similar to 

 that of the shark embryo. Consideration, therefore, is confined to the develop- 

 ment of the characteristic ganoid scale. 



In the formation of the ganoid scale of Lepisosteus, a different mechanism 

 is involved than in that of the placoid scale of the shark embryo. Most of 

 the scale is of dermal origin; the epidermal contribution of enamel substance 

 is small and restricted to the outer surface of the spines of the scale (fig. 

 266D-F). 



The scale first appears as a thin calcareous sheet, secreted by the dermal 

 cells in the outer portion of the dermis (fig. 266A). Unlike the formation of 

 dentine in the shark skin, the calcareous material comes to enclose some of 

 the scleroblasts (osteoblasts) or bone-forming cells (fig. 266B). This process 

 continues as the scale increases in mass, and the scleroblasts become dis- 

 tributed as bone cells within the hard, bony substance of the scale. These 

 cells occupy small spaces or lacunae within the bone-like substance, and small 

 canals (canaliculi) traverse the hard substance of the scale to unite with similar 

 canals from neighboring, bone-cell cavities (Nickerson, 1893, p. 123). 



Spine-like projections (fig. 266F) appear on the surface of the bony scales. 

 These spines are secondarily developed and form in a manner similar to the 

 placoid scale of the elasmobranch fish. That is, a dermal papilla is formed 

 externally to the already-formed dermal scale. This papilla pushes outward 

 into the epidermal layer, and a dentine-like substance appears on its outer 

 surface (fig. 266D). As development of the spine proceeds, this cap of dentine 

 gradually creeps basalward and unites secondarily with the dentine of the 



