THE SKELETON 



421 



Formation of Vertebral Articulations. In the course of develop- 

 ment the intervertebral discs 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 



Fig. 238. — Median sagittal section of the basis 



cranii and first three vertebral centra of an 



8-day chick. 



B. C, Basi-cranial 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. V. 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 vertebrae dorsally and ventrally (Schwarck). 

 Ossification is well advanced in the clavicles, long bones, 



