It is well known that when a piece of bone is excised and then buried in the tissues of an animal, new bone formation takes place in the transplant. Whether the transplant itself survives and proliferates or whether its presence merely acts as a stimulus to the surrounding tissue and a framework for the new bone is a disputed question with some evidence to support each hypothesis.
Formerly, the "metablastic" theory of bone formation prevailed; that is, the theory that various members of the connective tissue group possessed the power under certain circumstances or conditions of environment of changing from one to another. This theory has been generally displaced by the "neoblastic" theory which predicates a definite bone-forming cell, the osteoblast. According to this theory, the osteoblasts are formed either in the general developmental process through differentiation from indifferent mesenchymal cells, from osteoblasts which are present (periosteum and endosteum) through