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 
‘artilage, the superficial cells flatten over the surface of the 
‘artilage 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 
