DEVELOPMENT OF BACKBONE 



537 



To understand this skeletal axis, we must distinguish 

 clearly between the notochord and the backbone. 



The notochord is the first skeletal structure to appear in 

 the embryo. It arises as an axial differentiation of endo- 

 derm along the dorsal median wall of the embryonic gut or. 

 archenteron beneath the nerve-cord. The backbone, which 

 in most Vertebrates replaces the notochord, has a meso- 

 blastic origin. It develops as the substitute of the noto- 

 chord, and not from it, but from a skeletogenous sheath 

 surrounding it. 



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 the articular processes. 



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 tendinous region on the ventral 

 wall of the thorax. The sternum of some Reptiles, and of all Birds 

 and Mammals, arises from a cartilaginous tract uniting the ventral 

 ends of a number of ribs. 



Limbs and girdles. — The pectoral girdle consists of a 

 dorsal scapula, a ventral coracoid, and a forward growing 

 membrane-bone, the clavicle or collar-bone. 



According to Broom, frogs and some primitive Reptiles show a 

 coracoid and a pre-coracoid ; lizards and birds only a pre-coracoid ; 

 the Monotremes a coracoid and a pre-coracoid ; other mammals a 

 coracoid only. 



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

 a ventral posterior ischiac portion, with the articulation for 

 the limb between them, and of a ventral, usually anterior, 

 pubic portion. 



The fore limb — from Amphibians onwards — consists of a 

 humerus articulating with the girdle, a lower arm composed 

 of radius and ulna lying side by side, a wrist or carpus of 



