456 VERTEBRAL COLUMN OF AMPHIBIA. 



Ganoidei. In Acipenser and other cartilaginous Ganoids the 

 hoemal 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 vertebra?, 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 A; and k') neural and haemal arches 

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

 which, as mentioned on p. 452, 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 (GiJtte, 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 vertebrae. 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 pai*ts 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 embryo- 

 logically the Salamandridse present the most primitive type of forma- 

 tion of the vertebral column. 



It has already been stated that in Amphibia there is present 

 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 considerably shorter than the vertebral, 

 there is developed a ring-like cartilaginous thickening of the sheath, 

 which projects inwards so as to constrict the notochord. At a period 

 l)efore this thickening has attained considerable dimensions the noto- 

 chord becomes sufficiently constricted in the centre of each vertebral 

 region to give a biconcave form to the vertebrae for a very short 

 period of foetal life. 



