=Bone= consists of a series of lamellae of ossified substance between which are oval spaces, the =lacunae=, giving rise to numerous fine channels, the =canaliculi=, which radiate off in all directions. The lacunae are occupied by the =bone cells= which correspond to cartilage cells, from which if the bone is young, processes pass off into the canaliculi. It is obvious that the ossified substance of bone is intercellular in character, and corresponds to the matrix of cartilage. Bone may be compact, or loose and spongy in character, when it is known as =cancellous bone=. In compact bone many of the lamellae are arranged concentrically round cavities, the =Haversian canals=, which in life are occupied by blood-vessels. Each Haversian canal with its lamellae forms a =Haversian system=. In spongy bone instead of Haversian canals there occur large irregular spaces filled with marrow, which consists chiefly of blood-vessels and fatty tissue. The centre of a long bone is generally occupied by one large continuous marrow cavity. The whole bone is surrounded by a fibrous connective tissue membrane, =the periosteum=. THE DEVELOPMENT OF BONE. =Periosteal ossification.= An example of a bone entirely formed in this way is afforded by the parietal. The first trace of ossification is shown by the appearance, below the membrane which occupies the place of the bone in the early embryo, of calcareous spicules of bony matter, which are laid down round themselves by certain large cells, the =osteoblasts=. These osteoblasts gradually get surrounded by the matter which they secrete and become converted into bone cells, and in this way a mass of spongy bone is gradually produced. Meanwhile a definite periosteum has been formed round the developing bone, and on its inner side fresh osteoblasts are produced, and these with the others gradually render the bone larger and more and more compact. Finally, the middle layer of the bone becomes again hollowed out and rendered spongy by the absorption of part of the bony matter. =Endochondral ossification=[5]. This is best studied in the case of a long bone like the femur or humerus. Such a long bone consists of a shaft, which forms the main part, and two terminal portions, which form the =epiphyses=, or portions ossifying from centres distinct from that forming the shaft or main part of the bone. In the earliest stage the future bone consists of hyaline cartilage surrounded by a vascular sheath, the perichondrium.