THE SKULL. 377 



tube around the spinal cord. (2) In the notochordal sheath, 

 the formation of cartilage begins at regular intervals, corres- 

 ponding to the muscle segments or myomeres, or in other 

 words, to the primitive mesoderm segments, which used to 

 be called proto-vertebrae. (3) Finally, ossification may 

 occur, and a segmented backbone be established. 



In A7nphioxus, in Myxine, and in young lampreys (so- 

 called AinmocKtes), the notochord persists, unsegmented and 

 with a simple sheath. In the adult lamprey, there are rudi- 

 mentary arches of cartilage forming a trough in which the 

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

 Chimcera 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 con- 

 stricted 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. 



A vertebra generally consists of several more or less inde- 

 pendent 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 pro- 

 cesses 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 

 vertebras 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 Mam- 

 mals, 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 homo- 

 logous with branchial arches, while the primitive limbs are 

 made up of rnodified fin-rays originally like those of the 

 unpaired fins. According to Dohrn, the limbs are residues 



