422 THE DEVELOPMENT OF THE CHICK 
and membrane bones of the skull before it begins in the vertebra. 
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 
eA 4 
ptoute . 
Wage’ 
aoe 
1 Poe 
a5, 
Iria. 239. — Lateral sagittal section of the same vertebre (as in Fig. 
238). 
At. 1,2, Floor and roof of atlas. B.C., Basiscranii. Cerv.n. 1, 2, 
First and second cervical nerves. Med. Obl., Medulla oblongata. 
R. V. 2,3, 4, Ribs of the second, third, and fourth vertebra. YV. A. 
2, 3, Arches of the second and third vertebree. 
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 
