42 COMPARATIVE ANATOMY. 



also be remarked upon, as it differs in tailed and tailless Amphi- 

 bians. The long caudal portion of the tadpole's vertebral column, 

 which is very similar to that of Urodeles, undergoes during 

 metamorphosis a gradual retrogressive change, and the vertebrae 

 of its proximal end become fused together and ossified to form a 

 Jong unsegmented dagger-like bone, the urostyle (Fig. 32, Oc). 



The upper as well as the lower arches of the vertebras are in direct 

 connection with the centra. Lower arches are present only in the caudal 

 region of Urodeles, and evidently correspond to the already-mentioned basal 

 processes of the vertebrae of Ganoids. The most anterior in some cases 

 function as supports for the ribs, and this circumstance is sufficient to render 

 untenable the earlier view that the lower arches are modified transverse 

 processes 1 or fused ribs, as is the case in some Fishes. 



The neural spines, as well as the transverse processes, which are as 

 a rule bifurcated at the base and are present from the second vertebra onwards, 

 show the greatest variety as regards shape and size, differing in the several 

 regions of the body. The transverse processes of the single sacral vertebra, 

 which give attachment to the pelvis, are particularly strongly developed, 

 especially in the Anura. 



Articular processes (zygapophyses) which are usually 

 present in Fishes that possess a bony vertebral column, are well 

 developed in all Vertebrates from the Urodela onwards, and 

 consist of two pairs of projections arising respectively from the 

 anterior and posterior edges of the base of the neural arch. Their 

 surfaces are covered with cartilage, and overlap one another from 

 vertebra to vertebra like tiles on a roof: not unfrequently the 

 neural spines also articulate with one another, and thus a well- 

 articulated and mobile chain-like vertebral column results. 



From the Amphibia onwards the first vertebra, or so-called 

 atlas (and this is the only cervical vertebra of Amphibia), becomes 

 differentiated from the others. In Amphibians it consists of a 

 simple ring which articulates with the two condyles and the basis 

 cranii. As numerous researches have shown, however, the first ver- 

 tebra of Amphibians does not correspond to that (i.e. the atlas) of the 

 higher Vertebrates, but is much more nearly homologous with the 

 second cervical vertebra of the latter the axis (epistropheus). 

 This is demonstrated by a study of its development, which shows 

 that the real atlas loses its individuality as a separate mass, and 

 becomes united with the occipital region of the skull. 



Reptilia. In contrast to the numerous fossil forms, only a 

 few existing Reptiles, viz., Hatteria and the Geckos (Ascala- 

 bota) retain throughout life the primitive biconcave character of 

 their vertebrae, with the notochord expanded intervertebrally. 



In all the others, the notochord remains expanded longer in the 

 vertebral regions than intervertebrally, but in the adult it becomes 

 entirely aborted and replaced by bony tissue. This stronger and 

 more solid ossification of the whole skeleton forms a characteristic 



1 Traces of the transverse processes are present nearly to the end of the tail. 



