VEBTEBRAL COLU^IX 



35 



sJc.h 



sTi.i 



sh.i 



tr-.p 



Fig. 22.- 



A.- 



sJc-.l 



-Diagrams illdsteatixg the Development of the Notochokdal 

 Sheaths and Vertebral Column. 



B. 



C. 



-Early stage, showing notoehordal cells (hc) and primary sheath (nh^), as well 



as the mesoblastio skeletogenoiis layer {sk.l). 



-Later stage, in which the central notoehordal cells (nr) have become 



vacuolated and the peripheral cells have given ri.se to the " notoehordal epithe- 



limn" (rec. ep.) from which the fibrillar secondary sheath (sW) is derived : 



paired dorsal and ventral cartilages {d.a, i.a) have arisen in the skeletogenoiis 



layer. 



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



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



branohii : 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 (c), neural arch (jt. re), neural spine {n. sp), 

 transverse processes (tr.p) and articular processes (art); 



D 2 



