VERTEBRAL COLUMN 47 



occurs later in the vertebral axis. The cause of this segmentation 

 is to be traced primarily to the muscular system ; and it is evident, 

 on mechanical grounds, that the segmentation of the vertebral 

 column must alternate with that of the muscular segments or 

 myotomes. Small paired and segmentally arranged masses of 

 cartilage later appear in the skeletogenous tissue dorsally and 

 ventrally to the notochord, and these represent the rudiments of 

 the vertebrce (Fig. 34, B, E). This is the beginning of the second 

 or cartilaginous stage of the vertebral column ; and now ossification 

 may occur (bony stage). Those parts of the fibrous tissue which 

 do not become consolidated in this manner give rise to the ligaments 

 of the vertebral column. 



Two different modes of development of the vertebral column 

 from the above-mentioned dorsal and ventral cartilages or arcualia 

 may be observed. In Elasmobranchs and Dipnoans the secondary 

 notochordal sheath undergoes a fibrillar degeneration, and becomes 

 invaded by cartilage-cells from the arcualia which break through 

 the primary sheath at these points (Fig. 34, c) and gradually extend 

 so as to surround the notochord, thus forming a cartilaginous 

 sheath which may undergo segmentation to form a series of 

 vertebral bodies or centra ; the arcualia at the same time extend 

 dorsally and ventrally respectively to form the vertebral (neural 

 and haemal) arches. In other cases (e.g. Bony Ganoids, Teleosts, 

 Amphibians, and Amniota), the arcualia extend at their bases 

 round the notochord so as completely to surround it without 

 penetrating the primary and secondary sheaths (D). The centra 

 which are then formed by segmentation of this perichordal 

 cartilage may be described as perichordal centra, to distinguish 

 them from the chordal centra of Elasmobranchs and Dipnoans. 



During these differentiations of the skeletogenous tissue, the 

 notochord suffers a very different fate in the various Vertebrate 

 groups ; it may increase in size and persist as a regular cylindrical 

 rod, or it may become metamerically constricted by the formation 

 of the vertebral bodies, and even entirely disappear. In all Verte- 

 brates above Elasmobranchs, the embryonic vertebral column is 

 relatively shorter than the notochord around which it is formed, 

 and thus there has been a phylogenetic reduction in length of the 

 axial skeleton. 1 



Amphioxus. The notochord of Amphioxus exhibits many 

 primitive as well as special characters. It extends along the 

 whole length of the animal, whereas in the Craniata it always 

 ends anteriorly below the brain just behind the pituitary body. 



1 An ephemeral structure, the siibnotochordal rod or hypochorda, occurring in 

 embryos of Fishes and Amphibians, may be briefly referred to in this place. It 

 arises as a longitudinal furrow or ridge of the endoderm in the head and trunk, 

 on the dorsal side of the gut, with which it may for a time remain in connection, 

 but eventually becomes constricted off as a rod lying beneath the notochord. It 

 soon undergoes degeneration, but traces of it may persist as an elastic band. It 

 seems probable that this structure is a vestige of the epipharyngeal groove of 

 Amphioxus. 



