intervertebral body 



notochord canal 



AMPHICOELOUS 



OPISTHOCOELOUS PROCOELOUS 



(OR BICONVEX) 



BIPROCOELOUS HETEROCOELOUS 



(BIOPISTHOCOELOUS OR BIAMPHICOELOUS) 



;=?? 



TUBULAR AMPHIPLATYAN 



Figure 6-10. Diagrammatic representation of the various articular 

 surfaces and types of "centra" of vertebrae as seen in sagittal section, 

 with exception of heterocoelous type which has only the left posterior 

 half cut away. 



and a pair of similar bilateral ventroanterior nodules. These 

 have been likened, respectively, to the basidorsals and 

 interdorsals above, and to the basiventral of one vertebra 

 combined with the interventral of the next anterior verte- 

 bra below (see Figure 6-8 of Sphenodon and Figure 6-12). 

 This ventral pair of compound nodules forms the connective 

 tissue intercentrum of the adult vertebra. The blastema of 

 the rib is continuous with the basidorsal-interdorsal bulges, 

 and extends out into the septum between the myotomes. 

 Within the centrum itself, the cells are concentrically 

 arranged (around the central axis of the notochord). The 

 arrangement of cells in the centrum is apparent in both cross 

 and frontal sections. It is best marked at the center of the 

 vertebral body around the notochord and less clearly marked 

 outward and toward the ends of the centrum. 



Chondrification begins in the vertebral ring, and as pro- 

 cartilage extends through the invertebral rings. The result- 

 ing tube becomes segmented into vertebral blocks by move- 

 ment of connective tissue into the intervertebral regions. 

 The vertebral blocks are by now cartilaginous. The inter- 

 centra of the first and second cervicals and of the posterior 

 caudals chondrify separately in the procartilaginous tube. 

 Those of the caudal vertebrae later fuse with the centrum 

 of their respective vertebra. 



The neural arches chondrify at about the same time, the 

 cartilage extending upward to enclose the neural canal, and 

 laterally as transverse processes. The arches of adjacent ver- 

 tebrae meet above the spinal ganglia to form pre- and post- 

 zygapophyses. The neural arches soon fuse with the centra. 

 The ribs chondrify from separate centers. 



Ossification follows much as in the mammal. Centers 

 appear at the base of the neural arch, in the centrum, and in 

 the ribs. Ossification in the centrum is peculiar in that first 



perichordal tube 



vertebral ring 



blood vessel 

 spinal nerve 

 J_myotome ^ 



cranial sclerotomite 

 caudal sclerotomite 



ntervertebral ligament 



intervertebral body 



rib fused 



to interventral 



intervertebral disc 

 prospondylous ring 



opisthospondylous 

 ring 



Figure 6-11. Diagrammatic stages of development of bird vertebrae as described by 

 (1928). 



Piiper 



150 . THE VERTEBRATE BODY SKELETON 



