46 



COMPARATIVE ANATOMY 



B 



sk.l 



sh.1 



Ir.p 



nc. 



sk.l 



FIG. 34. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE NOTOCHORDAL 

 SHEATHS AND VERTEBRAL COLUMN. 



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

 as the mesodermic 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 

 epithelium " (nc. ep. ) from which the secondary sheath (sh 2 ) is derived : 

 paired dorsal and ventral cartilages, or arcualia (d.a, v.a) have arisen in the 

 skeletogenous layer (Cyclostomes, Cartilaginous Ganoids). 



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

 the secondary sheath (Elasmobranchii, Dipnoi). 



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

 gradually become reduced (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.sp), transverse processes (tr.p), and articular processes (art). 



and only broken through at the points of exit of the spinal 

 nerves. This stage is often known as the membranous stage, and 

 in it no indication is seen of the metameric segmentation which 



