COMPARATIVE ANATo.MV. 



traces of the \ettebral bodies and arches. This is the 

 second or cartilaginous stage of the vertebral column, and 

 now ossification may occur (bony stage). Those parts of the 

 fibrous tissue which do not become consolidated in this manner 

 give rise to certain ligaments of the vertebral column. 



During these differentiations of the skeletogenous tissue, the 

 notochord suffers a very different fate in the various Vertebrate 



j' 



Fie.. 20. TRA\"-vr.i:sr, Sr.rTmx OF TIIK VKI:TKI:]:AI. CULI'MN OF Ammoa 



C, Dotochonl ; C'.v, inner sheath, and 7>-, outer sheatli, of the notochord; SS, skele- 

 togeiions lavi-r ; f>/>, upper arch ; Ub, lo\\er arch ; I<] fatty tissue ; M, spinal 

 cord ; I', ]>i;i mater. 



groups ; it may increase in size and persist as a regular cylindrical 

 rod, or it may become constricted at definite intervals by the for- 

 mation of vertebral bodies, or even entirely disappear. 



During the cartilaginous and bony stages tin- various processes 



1 raii-\ rise, artirnlar processes, &C.) arc I'linm-if: and llir individual 



inav si'iiirtimrs li.'i'inni- t'li.-rd (n^i'tlicr, as for in.-tancr, in the regions <>f the 



iir.-k, .-ai-rnni. and 



All these ontogenetic stages find their exact |arallel in the 

 ]h\ Log^net ic dcvelo|iinent. of \'ci-tehrates, as the following JKI-VS 

 will show. 



Fishes. The vertebral column of all Fishes is distinguished by 

 a very uniform character of its elements, so that one can only 

 distinguish between trunk and caudal vertebrae. 



The mo-;t, eniliryonic type of notochord is seen in Amphioxus, 

 Amnioro'tes, ;md M\ \tnoids, in which it is entirely unseginenti'<l. 

 In the metamorphosed Petromyzon, cartilaginous elements ah-eady 

 make iln-ir appearance in the form of rudimentary arches and 

 spines, which do not meet above the spinal cord (comp. Kig. 40). 



