APPENDICULAR SKELETON. 433 



trough in which the spinal cord lies. In the cartilaginous Ganoid fishes, 

 in the Chimczra type, and in the Dipnoi, arches appear both above and 

 below, but yet there are no vertebral bodies. These begin in the 

 Elasmobranchs, in which the notochord is constricted by its encroaching 

 sheath. In the bony Ganoids the vertebrae are ossified, and so they are 

 in all the higher Vertebrates. Moreover, the notochord is more and 

 more completely obliterated as the backbone grows. 



It will be remembered (see p. 34) that according to 

 Kleinenberg the notochord supplies the necessary growth- 

 stimulus for the rise of its substitute, the backbone. 



A vertebra generally consists of several more or less 

 independent parts : the substantial centrum, the neural 

 arches which form a tube for the spinal cord, and are 

 crowned by a neural spine, the transverse processes which 

 project laterally, and are, perhaps, homologous with the 

 inferior haemal processes in the posterior region of Fishes 

 and some Amphibians. 



The ribs which support the body wall and usually arti- 

 culate with the transverse processes, or with the transverse 

 processes and centra, perhaps bear the same relation to the 

 vertebrae that the visceral arches do to the skull. 



In Amphibians for the first time a breast bone or sternum 

 is developed. It arises from two cartilaginous rods in a 

 tendinous region on the ventral wall of the thorax, and 

 seems to be different from that of higher animals. For the 

 sternum which is present in some Reptiles, and in all Birds 

 and Mammals, arises from a cartilaginous tract uniting the 

 ventral ends of a number of ribs. 



Appendicular Skeleton. 



No secure conclusion has yet been reached as to the 

 origin of the paired limbs. According to Gegenbaur, the 

 pectoral and pelvic girdles are homologous with branchial 

 arches, while the primitive limbs are made up of modified 

 fin rays, originally like those of the unpaired fins. 

 According to Dohrn, the limbs are residues of a longi- 

 tudinal series of segmentally arranged outgrowths, perhaps 

 comparable to the parapodia of an Annelid. According to 

 Wiedersheim, the girdle portion is primarily due to the 

 centripetal growth of the fin skeleton, which arose from 

 a localisation of the supports of continuous lateral folds. 



The pectoral or shoulder girdle consists of a dorsal 

 28 



