424 THE DEVELOPMENT OF THE CHICK 
The centers of ossification in the neural arches arise from 
the perichondrium a short distance above the body of the ver- 
tebra, and form bony rings about the cartilaginous arch. They 
gradually extend into all the processes of the neural arch. Thus 
the neural arches are separated from the vertebral centra by a 
dise of cartilage which is, however, finally ossified, fusing the 
arches and centra. At what time this occurs, and at what 
time endochondral ossification begins in the arches, is not 
known exactly for the chick. 
The vertebral column of birds is characterized by an extensive 
secondary process of coalescence of vertebre. Thus the two 
original sacral vertebre coalesce with a considerable number of 
vertebre, both in front and behind, to form an extensive basis 
of support for the long iliac bones. The definitive sacrum may 
be divided into an intermediate primary portion composed of 
two vertebrae, an anterior lumbar portion, and a posterior caudal 
portion. The development of these fusions has not been, appar- 
ently, worked out in detail for the chick. The bony centers are 
all separate on the sixteenth day of incubation (ef. Fig. 249). 
Similarly, the terminal caudal vertebra fuse to form the so-called 
pygostyle, which furnishes a basis of support for the tail feathers. 
Ill. DeVELOPMENT OF THE RIBS AND STERNAL APPARATUS 
In the membranous stage of the vertebral column, all of the 
trunk vertebra possess membranous costal processes the subse- 
quent history of which is different in different regions. In the 
cervical region these remain relatively short, and subsequently 
acquire independent centers of chondrification and ossification. 
The last two cervical ribs, however, acquire considerable length. 
In the region of the thorax, the membranous costal processes 
grow ventralward between the successive myotomes and finally 
unite in the formation of the sternum (q.v.). In the lumbar and 
sacral regions the membranous costal processes remain short. 
The primary costal process is an outgrowth of the membranous 
centrum, corresponding in position to the capitulum of the 
definitive rib. The tubereulum arises from the primary costal 
process while the latter is still in the membranous condition and 
crows dorsalward to unite with the neural arch in the region of 
the transverse process. (See Fig. 256.) 
The centers of chondrifieation and ossification of the typical 
