VERTEBRAL COLUMN. 463 



The notochord is the first skeletal structure to appear in 

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

 derm along the dorsal wall of the embryonic gut or 

 archenteron beneath the nerve - cord. The backbone, 

 which in most Vertebrates replaces the notochord, has a 

 mesoblastic origin ; it develops as the substitute of the 

 notochord, but not from it. 



In fialanoglossns, what is sometimes dignified with the name of 

 notochord, is restricted to the most anterior part of the body ; in the 

 Tunicata the notochord is confined to the tail, in Amphioxus it runs 

 from tip to tip of the body, in Cyclostomata and Dipnoi it persists as an 

 unsegmented gristly rod, in other Vertebrates it is more or less com- 

 pletely replaced by its better substitute the backbone. 



In Cyclostomata the notochord forms and is ensheathed by a cuticula 

 chordie (or membrana liinitans interna] ; outside this there is a meso- 

 blastic or skeletogenous sheath ; and outside this again lies a cuticula 

 sceleti or membrana liinitans externd]. It is likely that this represents 

 a primitive condition. What happens in most Vertebrates is that the 

 skeletogenous or mesoblastic sheath forms the backbone, and more or 

 less completely obliterates the notochord. The formation of cartilage 

 takes place at regular intervals in the notochordal sheath, and the 

 vertebral bodies thus formed alternate regularly with the primitive 

 muscle segments. This arrangement is necessary for the proper 

 attachment of the muscles to the future vertebra;, and makes it prob- 

 able, as we noticed above, that the segmentation of the backbone is 

 secondary, and was only acquired, as a mechanical necessity, when the 

 notochordal sheath became chondrified, and so rigid. Thus we reach 

 the conclusion that the primitive segmentation of the Vertebrates, alike 

 in head and trunk, finds its expression in the arrangements of the 

 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 vertebra, 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- 

 ca'fcs), 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 Chimera 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 en- 

 croaching sheath. In the bony "Ganoids" the vertebra? 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 Cosmopholis 

 mitchellt, the vertebrae are annular, as in some other ancient fishes. 



