254 report — 1846. 



logues of the ' cotes vertebrates ' of higher vertebrates, to which GeofFroy 

 assigned the name ' paraaux,' this appears to be another misapprehension of 

 the relations in question. 



Development of vertebra. — Before applying the idea of the archetypal 

 vertebra, or primary segment of the endo- skeleton, given in figs. 14 and 15, 

 to the elucidation of the modifications of those segments in the different ver- 

 tebrate classes, I shall premise a few observations on the mode of develop- 

 ment of the vertebrae in those classes. 



The chief condition of the development of distinct vertebrae in the trunk 

 is the conjunction of nerves with, or their progress from the spinal chord : 

 at least, this circumstance, with the concomitant exit of blood-vessels from 

 the neural canal, seems to determine the development of the neurapophyses : 

 and the vertebral bodies are not slow in coinciding in number Avith those im- 

 portant arches ; and in determining with the regular primary pairs of (inter- 

 costal, lumbar, &c.) arteries, the inferior or haemal arches. We 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 remains con- 

 tinuous and unjointed, because there is no co-extensive spinal chord giving 

 off pairs of nerves. 



An extremely delicate fibrous band, with successively accumulated gelati- 

 nous cells, compacted in the form of a cylindrical column, and inclosed by a 

 membranous sheath, is the primitive basis, called 'notochord'* {chorda dorsa- 

 lis seu gelatinosa, Lat., gcdlertsaule und ruckensaite, Germ.), in and around 

 which are developed the cartilaginous or osseous elements by which the 

 vertebral column is established in every class of Myelencephala. 



The earlier stages of vertebral development are permanently represented, 

 with individual peculiarities superinduced, in the lower forms of the class of 

 fishes-]-. In the Dermopteri or cyclostomous fishes, the neural and haemal 

 canals are formed by a separation of the layers of the outer part of the apo- 

 neurotic sheath of the gelatinous chorda : in the lancelet (Ampkioxus) there 

 is no distinction of structure in the cranial part supporting the anterior end 

 of the neural axis, with which the trigeminal, optic and olfactory nerves com- 

 municate, and the rest of the rudhnental vertebral column : a labial carti- 

 laginous arch supporting the tentacula is, at least, the only lineament of 

 development which sketches out the skull. In the myxinoids the skull in- 

 cludes a complex system of cartilages, but the vertebral column of the trunk 

 has not advanced beyond the gelatino-aponeurotic stage. In the lamprey 

 cartilaginous laminae are developed in the outer layer of the fibrous sheath, 

 and give the first indication of neural arches |. In the sturgeons (Sturio, 

 Polyodon) the inner layer of the fibrous capsule of the gelatinous notochord 

 has increased in thickness, and assumed the texture of tough hyaline carti- 

 lage. In the outer layer are developed distinct, firm, and opake carti- 

 lages, the neurapophyses, which consist of two superimposed pieces on each 

 side, the basal portion bounding the neural canal, the apical portion the 

 parallel canal filled by fibrous elastic ligament and adipose tissue ; above this 

 is the single cartilaginous neural spine. The parapophyses are now di- 

 stinctly developed, and joined together by a continuous expanded base, form- 

 ing an inverted arch beneath the notochord for the vascular trunks, even in 

 the abdomen. Pleurapophyses are articulated by ligament to the ends of the 



* Nwros back, xopc?}, string. We have hitherto had no English equivalent for this em- 

 bryonic keel or basis of every vertebrate animal : ' dorsal chord ' or ' chorda ' is liable to 

 be misunderstood for the ' spinal chord.' 



t Hunterian Lectures on Vertebrata, 1846, pp. 45, 46. 



J Cuvier, Memoires du Museum d'Histoire Naturelle, t. i. 1815, p. 130. 



