NO TOG HO RD AND VERTEBRAL COLUMN. 



457 



The stage with biconcave vertebrae is retained through life in the 

 Perennibranchiata and Gj'mnophiona. 



The chief peculiarity which distinguishes the later history of their 

 vertebral column from that of fishes 

 consists in the immense development 

 of the intervertebral thickenings just 

 mentioned, which increa:ie to such an 

 extent as to reduce the notochord, 

 where it passes through them, to a 

 mere band ; while the cartilage of 

 which they are composed becomes dif- 

 ferentiated into two regions, one be- 

 longing to the vertebra in front, the 

 other to that behind, the hinder one 

 being convex, and the anterior concave. 

 The two parts are not however abso- 

 lutely separated from each other. 



By these changes each vertebra 

 comes to be composed of (1) a thin 

 osseous somewhat hourglass-shaped 

 cylinder with a dilated portion of the 

 notochord in its centre, and (2 and 3) 

 of two halves of two intervertebral 

 cartilages, viz. an anterior convex half 

 and a posterior concave half. The vertebrae thus come to be 



Fig. 319. Vertical section 

 through the middle of a verte- 

 BRA OF Esox LUCIUS (Pike). (From 

 Gegenbaur.) 



ch. notochord ; cs. notochordal 

 sheath ; k. and k'. cartilaginous 

 tissue of the neural and haemal 

 arches ; /(. osseous haemal process ; 

 11. spinal canal. 



a? 



Fig. 320. Diagram representing the mode of development of the vertebra 

 IN the different types. (From Gegenbaur.) 



A. Ideal type in -which distinct vertebras are not established. 



B. Type of Pisces with vertebral constrictions of the notochord. 



C. Amphibian type, with intervertebral constrictions of the notochord by the 

 intervertebral parts of the cellular sheath. 



D. Intervertebral constriction of the notochord as effected in Eeptilia and Aves. 



E. Vertebral constriction of the notochord as effected in Mammalia, the interver- 

 tebral parts of the cartilaginous sheath being converted into intervertebral ligaments. 



c. notochord; cs. cuticular sheath of notochord ; s. cartilaginous sheath ; v. vertebral 

 regions; iv. intervertebral regions ; //. intervertebral joints. 



