sue, rib in the embryo. The third cervical has a free inter- 

 centrum. The fourth to last cervicals have ribs of increasing 

 size. The rib of the fourth cervical may be distinctly two- 

 headed, the capitulum, or ventral head, nearly touching 

 the intercentrum. The more posterior ribs have but a single 

 dorsal (tuberculum) head, which articulates increasingly 

 further back on the vertebral body. 



As there are no lumbar vertebrae, a series of dorsals com- 

 pletes the presacral part of this column (Figure 6-6). All of 

 the presacral vertebrae with the exception of the first 3 

 cervicals have ribs. Five or sLx of the ribs are complete with 

 costal segments attaching to the sternum, while 8 or 9 

 more have large irregular costal expansions associated with 

 the gastralia covering the abdominal area. Three more m- 

 complete ribs complete the presacral series of 25 vertebrae. 

 The ribs have uncinate processes like those in the bird. 



There are 2 sacrals similar to those of the lizard, and 29 

 caudals; the fourth and succeeding caudals, nearly to the 

 tip of the tail, have an intercentral arch (chevron bone). 

 The tail is like that of the lizard even to the breakage line 

 in the middle of the vertebral bodies of its terminal half 

 (Figure 6-6 E). The neural arches (without a spine) con- 

 tinue to the next to the last vertebra. 



EMBRYOLOGicAL DEVELOPMENT The development of the 

 vertebrae of Sphenodon appears to follow the same pattern 

 as in the mammal or bird (Figure 6-7). Sclerotomes form 

 and divide into sclerotomites; the posterior sclerotomite 

 unites with the anterior sclerotomite of the next posterior 

 segment to form a vertebral segment. Within the sclerotomic 

 tissue, a perichordal tube of circularly arranged cells is 

 formed around the notochord, and this is subdivided into 

 intervertebral and vertebral rings (Figure 6-8); the former 

 has more irregularly arranged and less spindle-like cells and 

 is joined with the rib and neural arch blastemas. 



In chondrification, centrum blocks are formed (Figure 

 6-7, 6-9) and are separated by connective tissue penetrating 

 inward from a line around the middle of the intervertebral 

 ring. The neural arches and ribs chondrify- separately, and 

 the rib joins the vertebra on the base of the neural arch. 



Ossification begins perichondrally (or perhaps subperi- 

 chondrally) in the neural arch, then in the vertebral centrum 

 (Figure 6-9) and the proximal part of the rib. 



The vertebrae remain amphicoelous (Figure 6-10), that 

 is, hollowed at both ends. The open ends of adjacent verte- 

 brae hold a ball-like intervertebral body with a remnant of 

 the notochord in its center. The intervertebral body is 

 reduced to little more than a disc between some vertebrae. 

 In the centrum the notochord first chondrifies and then be- 

 comes ossified. It is completely eliminated in the odontoid 

 process. 



Growth of the centrum and arch is probably a matter of 

 periosteal (membranous) extension outward accompanying 

 chondral ossification. Bony extension at the articular ends is 

 a matter of endochondral ossification. 



In describing the vertebrae of reptiles, the -form of the 

 articular ends of the vertebral bodies is frequently referred 

 to (Figure 6-10). The amphiplatyan vertebra is seen in the 

 mammal (that is, both articular surfaces of the centrum are 

 flat), the procoelous vertebra in the lizard and alligator, 

 and the amphicoelous vertebra in Sphenodon. Some lizards 

 have the articular surfaces just reversed: that is, the front 

 articulation is convex and the hind articulation is hollowed. 

 This condition is called opisthocoelous. The cervical verte- 

 brae of turtles are remarkable for the diversity of their 

 joints. The usual pattern is one in which the second and 

 third cervicals are opisthocoelous, the fourth biconvex, the 

 fifth and sixth procoelous, the seventh amphicoelous, and 

 the eighth biconvex, that is, rounded at both ends or just 

 the opposite of amphicoelous. In some cases, the articular 



notochord 



sclerotomic fissure (sclerocoel) 

 ectoderm 

 dermatome 

 yocoel 



ntersclerotomic 

 fissure 

 myotome 



notochord 



perichordal tube 



segmental artery, 

 segmental nerve 



perichordal port 

 arcual part 



vertebral body 

 bone 



-_1— muscle 



myoseptum 



and rib' 



rcuol 

 condensation 



intervertebral 

 body 



cartilage in notochord 



A B C 



Figure 6-7. Three diagrammatic stages in the somitic origin of vertebrae and ribs ,n Sphenodon. 



148 • THE VERTEBRATE BODY SKELETON 



