Section 71. in the Very Young Rabbit, the Limb Bones, Vertebral Column,

and many of the skull bones are simply plates and bars of cartilage; the future membrane bones, however are planned out in connective tissue. The development of the latter is simple, the connective tissue corpuscles functioning by a simple change of product as osteoblast. The development of the cartilage bones, however, is more complicated. Figure XVII., represents, in a diagrammatic way, the stages in the conversion of a cartilaginous bar to bone. To begin with, the previously sporadically-arranged (scattered anyhow) corpuscles (u.c.c.) are gathered into groups in single file, or in other words, into "columnar" groups (as at c.c.). The matrix becomes clouded with inorganic salts of lime, and it is then said to be calcified. This calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage. Simultaneous with the formation of the cavities (s.) due to this absorption, connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) grows into the ossifying* bar. It is from this connective tissue that the osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts. An osteoblast engaged in the absorption instead of the formation of bone is called an osteoclast.

* The formation of bone is called ossification. To ossify is to become bony.