408 THE DEVELOPMENT OF THE CHICK 



beyond, and they often represent the primordia of several future 

 bony elements. In such an area the cells are more closely aggre- 

 gated, the intercellular spaces are therefore smaller, and the 

 area stains more deeply than the surrounding mesenchyme. 

 There are, of course, stages of condensation in each case, from 

 the first vague and undefined areas shading off into the indifferent 

 mesenchyme, up to the time of cartilage or bone formation, 

 when the area is usually well defined. In most of the bones, 

 however, the process is not uniform in all parts; the growing 

 extremities may be in a membranous condition while cartilage 

 formation is found in intermediate locations and ossification has 

 begun in the original center of formation; so that all three stages 

 may be found in the primordium of a single bone (e.g., scapula). 

 Usually, however, the entire element is converted into cartilage 

 before ossification begins. 



The formation of cartilage (chondrification) is brought about 

 by the secretion of a homogeneous matrix of a quite special char- 

 acter, which accumulates in the intercellular spaces, and thus 

 gradually separates the cells; and the latter become enclosed in 

 separate cavities of the matrix; when they multiply, new deposits 

 of matrix form between the daughter cells and separate them. 

 As the original membranous primordium becomes converted into 

 cartilage, the superficial cells flatten over the surface of the 

 cartilage and form a membrane, the perichondrium, which be- 

 comes the periosteum when ossification takes place. 



The process of ossification in cartilage involves the following 

 stages in the chick: 



(1) Formation of Perichondral Bone. The perichondrium 

 deposits a layer of bone on the surface of the cartilage near its 

 center, thus forming a bony ring, which gradually lengthens into 

 a hollow cylinder by extending towards the ends of the cartilage. 

 This stage is well illustrated in Fig. 231 A and in the long bones 

 of Fig. 242; the bones of the wing and leg furnish particularly 

 good examples; the perichondral bone is naturally thickest in 

 the center of the shaft and thins towards the extremity of the 

 cartilages. 



(2) Absorption of Cartilage. The matrix softens in the 

 center of the shaft and becomes mucous, thus liberating the 

 cartilage cells and transforming the cartilage into the funda- 

 mental tissue of the bone marrow. This begins about the tenth 



