30 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



or matrix. The first step in bone formation in this manner consists 

 in the multiplication and enlargement of the cartilage cells and their 

 arrangement in rows at the center of ossification. The matrix in 

 which they are imbedded increases in quantity so that the cells 

 become farther separated from each other. A deposit of calcareous 

 material then takes place in this matrix, between the rows of cells, 

 so that they become separated from each other by columns of calcified 

 matrix. These columns are connected by transverse bars of cal- 

 careous substance. While this process is going on within the sub- 

 stance of the solid cartilage of which the developing bone consists, 

 certain changes are taking place on its surface. This is covered by a 

 very vascular membrane, the perichondrium, entirely similar to the 

 connective tissue layer, which forms the basis and constitutes the 

 periosteum of membrane bone. On the inner surface of the peri- 

 • chondrium the cells become osteoblasts or bone-forming cells, through 

 the agency of which a thin layer of bony tissue is being formed 

 between the outer membrane and the cartilage, in a manner not dis- 

 similar to that in which the formation of true membrane bone takes 

 place. The two processes above described go on simultaneously in 

 the development of cartilage bone. The second stage consists in 

 the prolongation into the cartilage of processes of the deeper or 

 osteogenetic layer of the perichondrium, these processes consisting 

 mostly of blood vessels and osteoblasts. In this way the bone is 

 gradually built up and finally reaches its adult condition. It vv'ill thus 

 be seen that practically the only difference betv/een a cartilage and a 

 membrane bone consists in the presence of a cartilaginous mold or 

 matrix, which precedes the former in the order of its development; 

 but it frequently happens that a cartilaginous mold may be present, 

 yet the resulting bone may be formed almost exclusively from the 

 perichondrium without involving the cartilage to but a very small 

 extent. There can be little doubt that if this subject were to be 

 followed up carefully all kinds of intermediate conditions would be 

 found connecting these processes of bone formation quite closely, save 

 and except the presence or absence of a perachondrial mold or matrix. 

 It frequently happens, moreover, that in some bones both processes 

 are concerned in its formation, and that one portion of the bone may 

 be formed by one method and another portion, in part, at least, by 

 the other.^ 



1 Huxley in his article on the Amphibia, quoted below, in speaking of the cranial bones of liana says: " The 

 ex-ocoipitais, prootics, and sphsn-cthmoid are ossifications which involve the chondro-cranium, though 

 they largely consist of secondary bone." And while he does not state directly that tliis is likewise true 

 of the quadrate, we are left to infer as much from his further statement. Again, in speaking of the long 

 bones, he says: " The long bones, both in the fore and hind limbs, consist of an axis of cartilage, sheathed 

 in, and more or less replaced by a diaphysis of membrane bone." We maj^ regard the para-sphenoid in 

 the same light, and while loosely spoken of as a "splint bone" or a "membrane bone," yet it is morpho- 

 logically an ossification primarily belonging to the chondro-cranium, developed in the perichondrium cover- 

 i ng its base and afterwards incorporated into the ossifications of the cartilage. 



