vertebral rin< 



^p-tyi-lneural tube 



'6'^t 



J V5U,,€=^^J^?^ » >■ ©1 — spinal nerve 









—basidorsal 

 -myoseptum 

 -interdorsal 



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"^i'f^1>°°°l|°' |li' ' i'>; Si intervertebral body 





Figure 6-8. A frontal section through a developing vertebra of 

 Sphenodon showing the blastemic condensations believed to be pres- 

 ent and which ore identified by Gadow's terms. (After Schauinsland, 

 1906) 



Of the many vertebrae in the synsacrum (10 to 15), none 

 can be identified as the sacrals, although in the developing 

 embryo two or more of these near the acetabulum (socket 

 for the femur) appear to be the original sacrals. The verte- 

 brae of the synsacrum have separately ossifying dorsal and 

 ventral rib heads lying lateral to the body, and these sup- 

 port the greatly elongated ilia of the pelvic girdle. 



The caudal vertebrae have long transverse processes; 

 intercenta are present and fused to the anteroventral mar- 

 gins of the more posterior members of this series. The 

 pygostyle represents about five fused vertebrae, the most 

 anterior of which is fairly distinct. 



In its vertebral column, the bird shows much shortening 

 as compared with the alligator or lizard. The shortening of 

 the caudal series and elimination of the lumbar series (in- 

 corporated into the synsacrum) is particularly interesting. 

 While the trunk and body are short, the neck is long. The 

 cervical series is elongated by the apparent process of mov- 

 ing the pectoral limb posteriorly. Shortening of the body 

 thus appears to involve lengthening of the neck segment at 

 the expense of the dorsals and shortening of the caudal seg- 

 ment. In birds and alligators, the number of presacral 

 vertebrae is about the same. 



surfaces are double (double convex, concave, or asymmetri- 

 cal); this is called ginglymoidy. 



&\rd The vertebral column of the bird has a marked 

 specialization of segments. There are 13 to 15 cervicals (up 

 to as many as 25 in the swan), 6 to 7 or more dorsals (sev- 

 eral of which may be fused), many sacrals, and a short series 

 of caudals (6 to 7) ending in a pygostyle (or terminal 

 structure) made up of several fused caudals (5 to 6). 



There is no preatlas vertebra (Figure 6-5 H). The atlas 

 is ring-like and the axis is like that of the lizard. The third 

 cervical has a short rib fused to the diapophysis and para- 

 pophysis. The more posterior cervicals have free ribs, in- 

 creasing in length up to that of the first thoracic. The last 

 cervicals and first thoracics have hypapophyses. The tho- 

 racic vertebrae bear large, two-headed ribs with uncinate 

 processes. The parapophysis is little more than a facet on 

 the line of juncture of the neural arch and body. 



EMBRYOLOGICAL DEVELOPMENT In its early stage, as ob- 

 served in a 96-hr chick (serial sections), the notochord is 

 proportionally much larger than in the mammal. It lacks a 

 distinct sheath and is enclosed in a continuous mass of 

 sclerotomic mesenchyme, more concentrated laterally, less 

 concentrated near the chord. The posterior scleromite is 

 the more dense. The cranial and caudal sclerotomites are 

 separated at first by an intrasegmental fissure. Around the 

 notochord, a distinct perichordal tube of circularly arranged 

 fusiform or spindle-shaped cells is formed (Figure 6-11 A). 

 This tube is constricted into vertebral segments by the 

 massing of mesenchyme cells in the intervertebral area. The 

 cells of this area are now denser than those of the vertebral 

 ring and are more irregularly disposed. The notochord is 

 dilated in the vertebral rings. 



As finally developed, the blastematic precursor of the verte- 

 bral includes 2 pairs of dorsal arcual masses in which the 

 polygonal (procartilaginous) cells are concentrically arranged 



cartilage in notochord 



occipital condyle 



dorsal perichordal bone center 



.cartilaginous centrum 



centrum] (C]) ^ 



intercentrum] (ici) 



ic4 ventral perichordal bone center 



Figure 6-9. A sagittal section through the anterior vertebrae of an embryo of Sphenodon showing 

 their cartilaginous precursors, and the first ossification. 



THE VERTEBRAL COLUMN • 149 



