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 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.
II. Mesoblastic.
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.
Bone may be formed in two ways:—
(1) by the direct ossification of pre-existing cartilage, when it is known as cartilage bone or endochondral bone;
(2) by independent ossification in connective tissue; it is then known as membrane or dermal or periosteal bone.