THE SKELETON 421 
Formation of Vertebral Articulations. In the course of develop- 
ment the intervertebral dises differentiate into a peripheral inter- 
vertebral ligament and a central suspensory ligament which at first 
contains remains of the notochord. There is a synovial cavity 
between the intervertebral and suspensory ligaments. This dif- 
ferentiation takes place by a process of loosening and resorption 
Fic. 238. — Median sagittal section of the basis 
cranii and first three vertebral centra of an 
8-day chick. 
B.C., Basi-eranial cartilage. iv. D. 1, 2, 3, 4, 
First, second, third, and fourth intervertebral 
discs. N.T., Floor of neural tube. s.n.b. 1, 2, 
First and second subnotochordal bars. VY. C. 
1, 2, 3, First, second, and third pleurocentra. 
of cells just external to the perichordal sheath (Fig.241). The in- 
tervertebral ligament takes the form of paired, fibrous menisci, or, in 
other words, the intervertebral ligaments are incomplete around 
the bodies of the vertebre dorsally and ventrally (Sechwarck). 
Ossification is well advanced in the clavicles, long bones, 
