464 STRUCTURE OF VERTEBRATA. 



The calcification in the notochordal sheath has simply formed a tube 

 around the notochord, --a state which illustrates an interesting persist- 

 ence of an embryonic phase. 



It will be remembered (see p. 38) 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 usually articulate 

 with the transverse processes, or with the transverse pro- 

 cesses and centra. 



Amphibians are the first to show a breast-bone or 

 sternum. It arises from two cartilaginous rods in a tendin- 

 ous 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. 



Limbs and girdles. 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 

 longitudinal 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 

 scapular portion or shoulder - blade, a ventral coracoid 

 portion, with the articulation for the limb between them, 

 and of a forward growing clavicle or collar-bone. 



The pelvic or hip-girdle consists of a dorsal iliac portion, 

 a ventral posterior ischiac portion, with the articulation for 



