VERTEBRAL COLUMN 



35 



B 



sk.l 



FIG. 22. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE NOTOCHORDAL 

 SHEATHS AND VERTEBRAL COLUMN. 



A. Early stage, showing notochordal cells (nc) and primary sheath (ah 1 ), as well 

 as the mesoblastic skeletogenous layer (sk.l). 



B. Later stage, in which the central notochordal cells (nc) have become 

 vacuolated and the peripheral cells have given rise to the " notochordal epithe- 

 lium" (no. ep.) from which the fibrillar secondary sheath (h?) is derived: 

 paired dorsal and ventral cartilages (d.a, i.a) have arisen in the skeletogenous 

 layer. 



C. Cartilage cells have passed through the primary sheath, and are invading 

 the secondary sheath (Cartilaginous Ganoids, Holocephali, Dipnoi, Elasmo- 

 branchii : in the last named chorda-centra are thus formed). 



D. The cartilages are growing round the notochord, outside its sheaths, which 

 gradually become reduced : thus arch-centra are formed (Bony Ganoids, 

 Teleostei, Amphibia, Amniota). 



A D represent the caudal region. 



E. A later stage in the development of a pre-caudal vertebra. The notochord 

 (nc) has become constricted, and the cartilages have united into a single mass 

 and have given rise to a centrum (<), neural arch (n.a), neural spine (n. *p), 

 transverse processes (tr.j)) and articular processes (art). 



D 2 



