432 STRUCTURE OF VERTEBRATA. 



The Vertebral Column. 



A dorsal skeletal axis is characteristic of Vertebrata, and 

 its usefulness is evident. It gives coherent strength to the 

 body ; it is usually associated very closely with a skull, with 

 limb girdles, and with ribs ; it affords stable insertion to 

 muscles ; its dorsal parts usually form a protective arch 

 around the spinal cord. 



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 wall of the embryonic gut or 

 archenteron. 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 Balanoglossits, 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 Amphioxiis 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 citticula 

 ihordtc (or Dienihrana Ihnitans interna) ; outside this there is a meso- 

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

 sceleti (or niembraiia limitans externa). 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 

 verteljral bodies thus formed alternate regularly with the primitive 

 muscle segments. This arrangement is necessary for the proper 

 attachment of the muscles to the future vertebrce, and makes it pro- 

 bable, 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. 



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

 the vertel)rai, 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 AiHphJoxus., in I\Iyxi]ic. and in young lampreys (known as Aninto- 

 ca'tes), the notochord persists, unsegmented and with a simple sheath. 

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



