NOTOCHORD AND VERTEBRAL COLUMN. 553 



A peculiar fact may here be noticed with reference to the formation 

 of the vertebral bodies in the tail of Scyllium, Raja, and possibly other 

 forms, viz. that there are double as many vertebral bodies as there are 

 myotomes and spinal nerves. This is not due to a secondary segmentation 

 of the vertebra; but, as I have satisfied myself by a study of the develop- 

 ment, takes place when the vertebral bodies first become differentiated. 

 The possibility of such a relation of parts is probably to be explained by 

 the fact that the segmentation of the vertebral column arose subsequently 

 to that of the nerves and myotomes. 



Ganoidei. In Acipenser and other cartilaginous Ganoids 

 the haemal and neural arches are formed as in Elasmobranchii, 

 and rest upon the outer sheath of the notochord. Since however 

 the sheath of the notochord is never differentiated into distinct 

 vertebras, this primitive condition is retained through life. 



Teleostei. In Teleostei the formation of the vertebral arches and 

 bodies takes place in a manner, which can be reduced, except in certain 

 minor points, to the same type as that of Elasmobranchii. 



There are early formed (fig. 314 k and k'} neural and haemal arches 

 resting upon the outer sheath of the notochord. The latter structure, 

 which, as mentioned on p. 549. corresponds to the cartilaginous sheath 

 of the notochord of Elasmobranchii, soon becomes divided into vertebral 

 and intervertebral regions. In the former ossification directly sets in 

 without the sheath acquiring the character of hyaline cartilage (Gotte, 419). 

 The latter forms the fibrous intervertebral ligaments. The notochord 

 exhibits vertebral constrictions. 



The ossified outer sheath of the notochord forms but a small part of 

 the permanent vertebne. The remainder is derived partly from an ossifi- 

 cation of the connective tissue surrounding the sheath, and partly from 

 the bases of the arches, which do not spread round the primitive vertebral 

 bodies as in Elasmobranchii. The ossifications in the tissue surrounding 

 the sheath usually (fig. 319) take the form of a cross, while the bases of 

 the arches (k and k') remain as four cartilaginous radii between the limbs 

 of the osseous cross. In some instances the bases of the arches also become 

 ossified, and are then with difficulty distinguishable from the other parts 

 of the secondary vertebral body. The parts of the arches outside the 

 vertebral bodies are for the most part ossified (fig. 319). In correlation 

 with the vertebral constrictions of the notochord the vertebral bodies are 

 biconcave. 



Amphibia. Of the forms of Amphibia so far studied 

 embryologically the Salamandridae present the most primitive 

 type of formation of the vertebral column. 



It has already been stated that in Amphibia there is present 



