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 

 disc 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 vertebrae. Thus the two 

 original sacral vertebra3 coalesce with a considerable number of 

 vertebrae, 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 (cf. Fig. 249). 

 Similarly, the terminal caudal vertebrae fuse to form the so-called 

 pygostyle, which furnishes a basis of support for the tail feathers. 



III. Development of the Ribs and Sternal Apparatus 



In the membranous stage of the vertebral column, all of the 

 trunk vertebrae 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 tuberculum arises from the primary costal 

 process while the latter is still in the membranous condition and 

 grows dorsalward to unite with the neural arch in the region of 

 the transverse process. (See Fig. 236.) 



The centers of chondrification and ossification of the typical 



