THE SKIN, AND ITS DERIVATIVES 



259 



is attached to the underlying bony plate, which forms the 

 so-called " isopedin " layer. This is the " cosmoid " scale. 

 In the primitive (extinct) sturgeons (the Palaeoniscoidea) 

 and in Polypterus, the layer of cosmin is not only covered by 

 bone underneath (the isopedin), but also on top, the superficial 



/ C °- d ' 



PC _^- ■*= 



Fig. 120. — Sections through the skin of Scyllium embryos, showing the 

 mode of development of the placoid scales or denticles. 



d, dentine ; e, ectoderm ; ec, modified ectoderm cells which produce the 

 enamel ; en, enamel ; m, mesoderm ; 0, odontoblasts, mesoderm cells 

 which produce the dentine ; pc, pulp-cavity. 



layer of bone being called the ganoin. This type of scale is 

 called palaeoniscoid. In Lepidosteus the structure of the scale 

 is similar, but the layer of cosmin has disappeared, and the 

 scale consists simply of a layer of ganoin overlying a layer of 

 isopedin. This is the lepidosteoid type of scale. The 

 palaeoniscoid and lepidosteoid scales are of course beneath 



