436 STRUCTURE OF VERTEBRATE. 



primitive segments and the nerves supplying these, and not in the 

 skeleton, which is a later development. 



In the higher Vertebrates, soon after the formation of the bodies of 

 the vertebrae, the rudiments of the neural arches appear in the mem- 

 brane surrounding the spinal cord. Finally, centres of ossification may 

 occur, and so produce the segmented backbone. 



In Amphioxus, in Myxine, and in young lampreys (known as Ammo- 

 cates), the notochord persists, unsegmented and with a simple sheath. 

 In the adult lamprey there are rudimentary arches of cartilage forming a 

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

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

 below, but there are as yet 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, as they are 

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

 more completely obliterated as the backbone grows. 



In the oldest known vertebral column in Britain, that of CosniopholU 

 7>iitchetti, the vertebrae are annular, as in some other ancient fishes. 

 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. 36) 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. 



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 



