338 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



The neural spine even when segmented in the cartilaginous 

 condition becomes ensheathed in a continuous layer of bone. 



In the case of Teleosts the cartilaginous stage of the haemal 

 arches is frequently completely eliminated from development, the 

 arches being laid down as bone in the connective tissue. 



In the Urodele Amphibia bone makes its appearance as a cell- 

 less sheath round the surface of the centrum, which gradually 

 increases in thickness and becomes cellular, enclosing connective- 

 tissue cells, and also spreads over the surface of the arcualia. The 

 cartilage becomes gradually absorbed and replaced by the bone. 



In Sphenodon, which may be taken as an example of the more 

 primitive Eeptiles, a bony sheath similarly develops round the 

 centrum, but according to Schauinsland it consists at first of a 

 distinct dorsal and ventral half. The bony tissue of the dorsal 

 portion spreads upwards so as to enclose the bases of the neural arch- 

 elements but the main portion of the arch-element on each side 

 becomes enclosed in an independent bony sheath of its own. This 

 latter appears first on the outer side of the cartilaginous arch and 

 may persist as a separate bony element for a long period, even 

 throughout life in the Crocodiles and various other Eeptiles. 



Bone formation also spreads inwards into the substance of the 

 cartilaginous centrum along what possibly corresponds to the 

 boundary between the primary centrum and the chondrified tissue 

 external to it (Fig. 152, p. 302). Thus arises a deep-seated centre of 

 active bone formation. 



From these various centres ossification spreads, the cartilage 

 being gradually supplanted by bone. Not the whole of the bone so 

 deposited is permanent : a great part of that lying outside the 

 primary centrum becomes again absorbed, leaving a superficial tract 

 connected with the more central portion only by sparse bony trabe- 

 culae, the meshes being occupied by intrusive connective tissue. 



An interesting adaptive feature is found in the tail region of 

 certain Eeptiles (Lizards, Sphenodon) which enables the possessor to 

 break off its tail suddenly when seized by an enemy. In these 

 animals the halves of the centrum derived from successive sclero- 

 tomes have reverted to a condition of incomplete fusion — the ossifica- 

 tion being more or less interrupted in the plane of contact of the 

 two successive sclerotomes by a transverse septum of cartilage. As 

 at the same time the corresponding connective-tissue septum between 

 consecutive myotomes remains weaker than usual a violent contrac- 

 tion of the caudal muscles is able to tear across both cartilaginous and 

 connective-tissue septum and break off the distal portion of the tail. 



In the case of the Birds it would appear that the main centre of 

 ossification of the centrum corresponds to the deep-seated one in 

 Sphenodon, the superficial bone-forming activity being much reduced. 

 A characteristic feature of the Birds, associated primarily with their 

 peculiar respiratory movements, is the extensive fusion which takes 

 place between the vertebrae of the trunk region. 



