422 THE DEVELOPMENT OF THE CHICK 



and membrane bones of the skull before it begins in the vertebrae. 

 It takes place in antero-posterior order, so that a series of stages 

 may be followed in a single embryo (cf. Fig. 242). There are 

 three main centers for each vertebra, viz., one in the body and 

 one in each neural arch. The ossification of the centrum is almost 



>cerv.Ml 



FIG. 239. Lateral sagittal section of the same vertebrae (as in Fig. 

 238). 



At. 1, 2, Floor and roof of atlas. B. C., Basis cranii. Cerv. n. 1, 2, 

 First and second cervical nerves. Med. Obi., Medulla oblongata. 

 R. V. 2, 3, 4, Ribs of the second, third, and fourth vertebra?. V. A. 

 2, 3, Arches of the second and third vertebrae. 



XII 2, Second root of hypoglossus. 



entirely endochondral, though traces of perichondral ossification 

 may be found on the ventral and dorsal surfaces of each centrum 

 before the endochondral ossification sets in. The perichondral 

 centers soon cease activity. The endochondral centers arise 

 just outside the perichordal sheath near the center of each ver- 

 tebra on each side of the middle line, but soon fuse around the 



