THE VEUTEBRit: IN FISHES. 45 



cranial nerves ; and they undergo, in their neural arches, as extreme 

 modifications as we perceive in the htemal portions of those vertebrae 

 that protect the great centres of the vascular system. AVe may 

 learn how much the development of the neurapophyses and vertebral 

 bodies depends, in the trunk, upon the conjunction of nerves with the 

 spinal chord, by the fact that, in the regenerated tails of lizards, the 

 vertebral axis i*emains continuous and unjointed, because there is no 

 co-extensive spinal chord giving oif pairs of nerves. 



An extremely delicate fibrous band, with successively accumulated 

 gelatinous cells, compacted in the form of a cylindrical column, and 

 inclosed by a membranous sheath, is the primitive basis, called 

 ' chorda dorsalis,' in and around which are developed the cartila- 

 ginous or osseous elements, by which the vertebral column is estab- 

 lished in every class of Myelencephala (iii. p. 340.). 



The earlier stages of vertebral development are permanently re- 

 presented, with individual peculiarities superinduced, in the lower 

 forms of the class of fishes. In the anencephalous Lancelet {Bran- 

 chiostomd) the lowest of all, the entire vertebral column consists of 

 the gelatino-cellular chord and its membranous sheath. In the 

 Lamprey cartilaginous arches and spines are added above the chorda 

 dorsalis, in the membranous wall of the neural canal, and in the tail, 

 also beneath it. In the Sturgeon and Chimaera, the bases of the 

 cartilaginous arches inclose the ' chorda.' In the Lepidosiren the 

 neural and hcemal arches and their spines are ossified, but the 

 centrums are still confluent as a dorsal membrano-gelatinous chord. 

 In many Sharks and Rays the ' chorda ' is encroached upon by 

 osseous or cartilaginous convergent laminse, and by concentric, 

 successively shorter, centripetally developed cylinders, and is thus 

 reduced to a string of gelatinous beads, each bead occupying 

 the interspace between the opposed concave surfaces of the ver- 

 tebral bodies. This moniliform state of the chorda dorsalis is 

 persistent in most osseous fishes, the biconcave bodies of the vei-- 

 tebrae being perforated in the centre ; whilst, in some other osseous 

 fishes, the gelatinous biconical segments of the ' chorda ' are insulated 

 by the completed centripetal progress of ossification ; and in one ex- 

 ception (Lepidosfens), they are converted into osseous balls, fixed to 

 the fore part of each vertebral body, which plays in the concavity or 

 cup of the next vertebra in advance. 



The neural and htemal arches and spines ai'e bony in all osseous 

 fishes ; and in all fishes chondrification and ossification of the ver- 

 tebral column commences in these arches. In reptiles, birds, and 

 mammals, the vertebras are bony throughout. 



Development diverges from the membrano-gelatinous stage, so as 



