554 



VERTEBRAL COLUMN OF AMPHIBIA. 



around the notochord a cellular sheath, equivalent to the 

 cartilaginous sheath of Elasmobranchii. In the tissue on the 

 dorsal side of this sheath a series of cartilaginous processes 

 becomes formed. These processes are the commencing neural 

 arches ; and they rest on the cellular sheath of the notochord 

 opposite the middle of the vertebral regions. 



A superficial osseous layer becomes very early formed in 

 each vertebral region of the cellular 

 sheath ; while in each of the inter- 

 vertebral regions, which are con- 

 siderably shorter than the vertebral, 

 there is developed a ring-like carti- 

 laginous thickening of the sheath, 

 which projects inwards so as to 

 constrict the notochord. At a 

 period before this thickening has 

 attained considerable dimensions 

 the notochord becomes sufficiently 

 constricted in the centre of each 

 vertebral region to give a biconcave 

 form to the vertebrae for a very 

 short period of fcetal life. 



The stage with biconcave vertebra; is 

 retained through life in the Perennibran- 

 chiata and Gymnophiona. 



FIG. 319. VERTICAL SECTION 

 THROUGH THE MIDDLE OK A VER- 

 TEBRA OF Esox LUCIUS (PIKE). 

 (From Gegenbaur.) 



ch. notochord ; cs. notochordal 

 sheath ; k. and /'. cartilaginous 

 tissue of the neural and haemal 

 arches ; h. osseous hoemal process ; 

 n. spinal canal. 



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 increase 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 differentiated 

 into two regions, one belonging 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 absolutely 

 separated from each other. 



By these changes each vertebra comes to be composed of (i) a 

 thin osseous somewhat hourglass-shaped cylinder with a dilated 

 portion of the notochord in its centre, and (2 and 3) of two 



