8 THE VERTEBRATE SKELETON. 



The scales of fish are wholly or in part mesoblastic 

 in origin, being totally different from those of reptiles. The 

 cycloid and ctenoid scales of Teleosteans (see p. 105) are thin 

 plates coated with epidermis. They are sometimes bony, but 

 as a rule are simply calcified. Ganoid scales are flat plates 

 of bone coated with an enamel-like substance, and articulating 

 together with a peg and socket arrangement; they are pro- 

 bably identical with enlarged and flattened placoid scales. 



The armour plates of fossil Ganoids, Labyrinthodonts, 

 and Dinosaurs, and of living Crocodiles, some Lizards and 

 Armadillos, are composed of bone. They are always covered 

 by a layer of epidermis. 



The antlers of deer are also composed of bone ; they 

 will be more fully described in the chapter on mammals. 

 It may perhaps be well to mention them here, though they 

 really belong to the endoskeleton, being outgrowths from the 

 frontal bones. 



B. ENDOSKELETAL STRUCTURES. 

 I. HYPOBLASTIC. 



(a) The notochord is an elastic rod formed of large 

 vacuolated cells, and is surrounded by a membranous sheath 

 of mesoblastic origin. It is the primitive endoskeleton in 

 the Chordata, all of which possess it at some period of their 

 existence; while in many of the lower forms it persists through- 

 out life. Even in the highest Chordata it is the sole repre- 

 sentative of the axial skeleton for a considerable part of the 

 early embryonic life. A simple unsegmented notochord 

 persists throughout life in the Cephalochordata, Cyclostomata, 

 and some Pisces, such as Sturgeons and Chimaeroids. 



(b) The enamel of the pharyngeal teeth of the Salmon 

 and many other Teleosteans is hypoblastic in origin. The 

 epiblast of the stomodaeum, in which the other teeth are 

 developed, passes into the hypoblast of the mesenteron in 

 which these pharyngeal teeth are formed. 



