SCALES 



215 



is covered with isolated placoid scales, it is supposed that the 

 denticles developed extensive basal plates in the dermis ; by the 

 coalescence of adjoining basal plates were formed larger plates or 

 scales studded on their outer surface with small denticles. The 

 modification, spreading out, or fusion of the denticles then gave 

 rise to the ganoine, and the ornamentation of tubercles and ridges 

 found on the surface. 



This theory is open to several objections. Firstly, the ' placoid ' 

 denticle is never found separate from its basal plate, which is merely 



skn 



A. 



Fio. 188. 



Lepidosteus osseus, I* A, surface view of trunk-scales from which the skin and connective 

 tissue have been removed on the left side. B, longitudinal section through the skin and 

 scales, a.p, anterior articulating process; c.t, connective tissue of cutis ; d, denticle; d.p, 

 dorsal articulating process ; ep, epidermis ; </, ganoine layer ; ck, scale ; sku, skin covering 

 scales ; v.c, -vascular canal. 



an extension of it inwards. In the Osteichthyes, on the contrary, 

 denticles are developed separately, often later than the underlying 

 bony plate, and only secondarily become connected with it. 

 Secondly, the basal plate of a placoid scale is never formed of true 

 bone, but either of dentine or of some allied substance. Thirdly, 

 the scale often sinks away from the epidermis, and grows by the 

 addition of new layers over its surface (cp. p. 356). The shiny 

 ganoine layer of the scales of the Osteichthyes, being formed by the 

 mesoblast, must not be confused with true enamel deposited by the 

 epidermis. 



