443 



"All these processes are connected with the sheath, but not with the 

 core of the notochord." 



As development advances, the ring-like segments increase in breadth, 

 length and thickness ; at the same time they become somewhat cartilagi- 

 nous, and then ossify. Each widens somewhat more at its ends than in 

 the middle, and so appears a little contracted in the centre. It is only 

 after birth that such an internal thickening takes place as to interrupt 

 the cavity of the vertebral centrum. 



The sheath of the notochord is originally of one texture throughout, but 

 the smaller portions, which lie between the vertebral centra, assume a 

 fibrous texture, contemporaneously with the appearance of the latter. The 

 included substance of the notochord loses its peculiar dense and elastic 

 character, becomes first gelatinous, then grumous, and finally resembles a 

 thick serum. 



The crura of the upper and lower vertebral arches (in the tail) unite 

 in pairs, and their points of union grow out into spinous processes. 



The ossification of the processes which arise from the vertebrae com- 

 mences at the point of junction of the process with the centrum. " A 

 small bony point arises, which appears to belong to both centrum and pro- 

 cess, and from whence ossification extends into both. In each vertebral 

 centrum, therefore, as well of the tail as of the trunk, ossification pro- 

 ceeds from different and distant points." 



2. Cyprinus bticca(Von Bar, 'Untersuchungen iiber die Entw. d. Fische.' 

 1835). 



" At the end of the first day the notochord is covered by something 

 which surrounds it like thin plates ; these are the developing bodies of 

 vertebrae. It is clearly observable that these bodies of vertebrae are not 

 undivided rings surrounding the notochord, but that they consist of many 

 pieces united by sutures. This condition also is persistent in the stur- 

 geons. The body of the vertebra, therefore, is formed of the coalescence 

 of many pieces, and a lateral suture seems to indicate that these pro- 

 cesses are elongations of the previously observed upper and under verte- 

 bral arches." 



Von Bar imagines that the unconstricted part of the notochord gives 

 rise to the intervertebral ligaments. 



From these observations of Rathke and Von Bar, it would appear as if 

 the annular ossifications which surround the notochord arose by the co- 

 alescence of ossific centres, primarily developed at the junction of the apo- 

 physes with the centra. My own observations on Gasterosteus, however, 

 show, like those of Vogt on Coregonus, that the centra ossify from distinct 

 rings deposited immediately round the notochord ; and I am very strongly 

 inclined to believe that the corresponding primary annular diaphyses of 

 the vertebras in Cyprinus and Blennius have been overlooked. 



3. Coregonus palea (Vogt, 'Embryologie des Saumones/ 1842, p. 104 et 

 seq.). "But it is necessary to distinguish carefully between what we call 

 vertebrae in the adult fish, that is to say, those osseous or cartilaginous 

 pieces intended for the support of the whole body, and more particularly 

 of the spinal cord, and such vertebral divisions as we find in embryos. These 



