VERTEBRAL COLUMN. 435 



The great development of the muscle segments of the trunk region 

 induces a secondary segmentation of the mesoblastic skeleton (vertebral 

 column), while the slight development of the muscles of the head region 

 exercises no such influence upon its skeleton ; this is therefore always 

 quite devoid of segmentation. The segmentation of the head, in con- 

 tradistinction to the skull, is expressed, although indistinctly, by the 

 muscle segments and by the nerves supplying these, perhaps also by 

 the lateral sense organs, the ganglia, and the arches. While it is quite 

 certain that it is the head that is segmented and not the skull, the 

 details of the segmentation are still much debated. 



Vertebral column. — A dorsal skeletal axis is character- 

 istic 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 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 Balanoglossus, 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 

 chorda (or membrana limitans interna) ; outside this there is a meso- 

 blastic 01 skeletogenous sheath ; and outside this again lies a cuticula 

 sceleti (or membrana 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 

 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 vertebrae, 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 



