452 



cation which takes place in the immediate neighbourhood of the notochord 

 and in the neural and haemal arches, The intercentra correspond in fishes, 

 as in Amphibia and birds, to the middle of each somatome ; and it does 

 not appear that they are, to any appreciable extent, produced by the 

 metamorphosis of the notochord. 



When ossification takes place, the diaphysis appears as a ring, or part of 

 a ring, in immediate contact with, or very close proximity to, the noto- 

 chord, which it usually embraces completely, though in the rare case of 

 some Amphibia, only partially. The diaphysis then increases inwards so 

 as to constrict the notochord, and outwards, so as to invade the centrum 

 more and more. A distinct ossification is commonly formed in each 

 neural arch, and one or more others in each haemal arch. 



In the higher or abranchiate oviparous Vertebrata there would seem to 

 be no other centres of ossification in the vertebra than the five just men- 

 tioned, except those of the terminal epiphyses. In fishes, on the other 

 hand, a distinct centre which might be termed the paraphysis is occa- 

 sionally found in the paraphysial portion of the centrum. 



The dorsal vertebrae of a young carp exemplify this structure remarkably 

 well. The diaphysis is represented by an annular osseous ring, which 

 surrounds the notochord and gives off a vertical median process or plate, 

 and two inferolateral plates, which unite the hollow bony cones into 

 which the osseous ring dilates in front and behind. The ossified neur- 

 apophyses expand into wedge-like lower ends, which embrace the vertical 

 plate of the diaphysis, and whose apices come into contact with its 

 annular part. 



A thick cuneiform mass is interposed between the base of the neurapo- 

 physis and the inferolateral plate of the diaphysis on each side. The 

 outer surface forms part of the general contour of the vertebra, and is not 

 produced into a distinct process, though it represents a parapophysis, and 

 gives attachment to the broad head of the distinctly ossified rib. The 

 outer half of the mass is ossified as a distinct paraphysis ; the inner in 

 the young carp is still cartilaginous. In the adult the whole wedge-like 

 paraphysial portion of the centrum is ossified ; but, instead of becoming 

 united with the diaphysis and neurapophysis, it is anchylosed with the 

 rib, and seems to form its head. In the pike, the paraphysis, more or less 

 produced into a parapophysis, remains distinct from both rib and dia- 

 physis, and the latter occupies a very much larger share of the whole 

 vertebra. 



As I have said above, no distinct ossific centre appears to be developed 

 in the paraphysial region of the centrum in any of the abranchiate Verte- 

 brata ; but it becomes ossified partly by the encroachment of the neur- 

 apophysial, and partly by that of the diaphysial, ossifications. 



These two ossifications may coalesce so as to leave no trace of their 

 primitive distinctness, as in Ophidia, Lacertilia, and birds ; or as in mam- 

 mals, Crocodilia, Chelonia, and many extinct reptiles, they may remain 

 for a long time, or permanently, separated by a suture, which may be 

 termed the "neurocentral suture" 



It is very commonly assumed that this neurocentral suture is a sort of 



