(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=.
With the exception of the _clavicle_[4] all the bones of the trunk and limbs, together with a large proportion of those of the skull, are preformed in the embryo in cartilage, and are grouped as cartilage bones; while the clavicle and most of the roofing and jaw-bones of the skull are not preformed in cartilage, being developed simply in connection with a membrane. Hence it is customary to draw a very strong line of distinction between these two kinds of bone; in reality however this distinction is often exaggerated, and the two kinds pass into one another, and as will be shown immediately, the permanent osseous tissue of many of those which are generally regarded as typical cartilage bones, is really to a great extent of periosteal origin. The palatine bone, for instance, of the higher vertebrates in general is preceded by a cartilaginous bar, but is itself almost entirely a membrane bone.
Before describing the development of bone it will be well to briefly describe the structure of adult bone and cartilage.
The commonest kind of =cartilage=, and that which preforms so many of the bones of the embryo, is =hyaline= cartilage. It consists of oval nucleated cells occupying cavities (=lacunae=) in a clear intercellular semitransparent matrix, which is probably secreted by the cells. Sometimes one cell is seen in each lacuna, sometimes shortly after cell-division a lacuna may contain two or more cells. The free surface of the cartilage is invested by a fibrous membrane, the =perichondrium=.
=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.