=Bone= is in many cases exoskeletal, but it will be most conveniently described with the endoskeleton.
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 probably 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.
(_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 throughout life. Even in the highest Chordata it is the sole representative 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.
The most primitive type of a mesoblastic endoskeleton consists of a membranous sheath surrounding the notochord, as in _Myxine_ and its allies. The first stage of complication is by the development of cartilage in the notochordal sheath, as in _Petromyzon_. Often the cartilage becomes calcified in places, as in the vertebral centra of _Scyllium_ and other Elasmobranchs. Lastly, the formation of bone takes place; it generally constitutes the most important of the endoskeletal structures.