xx] 



AETHEOSPONDYLI 



497 



are at first cartilage, but become ossified. The dorsal and ventral 

 intercalaries belonging to the same myotome become connected 

 by an intervertebral ring of cartilage which constricts and 

 finally obliterates the notochord between the centra. Then this 

 ring undergoes absorption in its centre, so that a joint cavity 

 is formed (synovial cavity) arid the front half, which is concave, 

 becomes attached to the vertebra in front and ossified, and the 



Fio. 242. 





Three stages in the development of the vertebral column of 

 Lepidosteus. 



Stage in which basidorsals, basiventrals, and intercalaries are all separate. 

 Stage in which basidorsals and basiventrals are connected by a bony ring, 

 and in which the intercalaries have fused to form a cartilaginous ring in 

 which a space, the rudiment of the synovial cavity, is appearing. 

 Stage of the completed vertebra. 



Basidorsal. 2. Notochord. 3. Basiventral. 4. Dorsal inter- 

 calary. 5. Ventral intercalary. 6. Bony ring connecting basi- 

 dorsal and basiventral. 7. Synovial cavity. Dotting, cross-hatching, 

 etc. as in Fig. 232. 



hinder half, which is convex, becomes attached to the vertebra 

 behind, and likewise ossified, i.e. replaced by bone, so that it comes 

 about that the vertebrae acquire articular surfaces by which they can 

 glide on one another. If this description has been followed, it will 

 be seen that a vertebra consists of the basidorsal and the smaller 

 part of the dorsal intercalary belonging to one myotome, and of the 

 basiventval and the larger part of the ventral intercalary belonging 

 s. & M. 32 



