The derivation of tissues from more rudimentary antecedents, which is the normal method of achieving adult structures in the human body, constitutes a major source for tumor pathology, since it is in such developmental steps that most neoplasms arise. The structure of the tumor often reveals this fact by a cycle of changes similar in form and sequence to the succesive stages of normal growth in the portion of the body where it arises.
This is well illustrated by the relation of certain tumors of the bone to the evolution and embryologic development of the skeleton. In its primitive form the early vertebrate spine (notochord) is membranous, composed of a cellular connective tissue which is distended with fluid to render this structure resistant to pressure. Later, the notochord becomes cartilaginous, to be replaced in higher forms by a vertebral column and skeleton composed nearly entirely of bone (Darwin,1 Lull