34 COMPARATIVE ANATOMY. 



traces of the vertebral 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 



FIG. 20. TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Ammocodes. 



C, notochord ; Cs, inner sheath, and Ee, outer sheath, of the notochord ; SS, skele- 

 togenons layer ; Ob, upper arch ; lib, lower arch ; F, fatty tissue ; M, spinal 

 cord ; P, pia 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 the various processes (spinous, 

 transverse, articular processes, &c.) are formed : and the individual vertebras 

 may sometimes become fused together, as for instance, in the regions of the 

 neck, 'sacrum, and coccyx. 



All these ontogenetic stages find their exact parallel in the 

 phylogenetic development of Vertebrates, as the following pages 

 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 most embryonic type of notochord is seen in Amphioxus, 

 Ammoccetes, and Myxinoids, in which it is entirely unsegmented. 

 In the metamorphosed Petromyzon, cartilaginous elements already 

 make their appearance in the form of rudimentary arches and 

 spines, which do not meet above the spinal cord (comp. Fig. 49). 



