DEVELOPMENT OF THE ENDOSKELETON 685 



Fig. 321. Development of vertebrae. The vertebral column in the phylum Vertebrata 

 is a variable structure. In the early embryo the primitive notochord serves as the primitive 

 axis. Later this structure develops fibrous sheaths in fishes and amphibia. The notochord 

 plus its surrounding sheaths serves as the only axial support in the embryo and adult 

 stages of Amphioxus and Cyclostomes. However, in all true vertebrates, the notochord 

 is supplemented during later embryonic stages by vertebral rudiments known as arcualia 

 (fig. 321, A). Eight arcualia are present typically in each vertebral segment. The arcualia 

 begin as mesenchymal condensations from the sclerotome (see fig. 252, A-D), and later 

 are transformed into cartilaginous masses. In the elasmobranch fishes the cartilaginous 

 arcualia fuse to form the vertebra as described below, but in most vertebrates they 

 undergo ossification. 



I. The Formation of Vertebrae in Fishes. In certain instances among the fishes, the 

 arcualia are merely saddled on to the notochord and its sheaths. This condition is found, 

 for example, in the lung fishes and cartilaginous ganoid fishes (fig. 321, E). A vertebral 

 centrum is not developed in these instances. 



In the elasmobranch fishes the vertebra is formed essentially from that group of 

 arcualia known as the basalia, that is, the basidorsals and basiventrals. These rudi- 

 ments invade the fibrous sheath from above and below on either side and form the 

 neural arch and centrum as indicated in fig. 321, C. The interbasalia — that is, the 

 interdorsals and interventrals— lie between the vertebrae. The notochord is constricted 

 greatly in the region of the centrum but is disturbed little in the areas between the 

 centra. That is, the centrum is hollowed out or deeply concave at either end. This form 

 of centrum is found in all amphicoeious vertebrae (fig. 321, P). In the tail region 

 (fig. 321, C), there are two vertebrae per muscle segment. This condition is known as 

 diplospondyly. Other cartilaginous elements may enter into the formation of the centrum 

 as indicated in fig. 321, C. 



The diplospondylous condition in the tail region of Ainia presumably is developed 

 as indicated in fig. 321, H'. In the trunk region of Ainiu the arcualia associate to form 

 the vertebrae as in fig. 321, H. A certain amount of membrane bone may enter into the 

 composition of the centra in Ainiii. In the teleost fishes (fig. 321, I), the basidorsals 

 form the neural arches, but the centrum is developed almost entirely from the ossifi- 

 cation of fibrous connective tissue membrane (i.e., membrane bone formation). The 

 basiventrals form the area of attachment of the pleural ribs and also form the hemal 

 arches. 



II. Development of Vertebrae in Amphibia. In the frog (fig. 321, B), the neural 

 arch of each vertebra appears to arise as the result of fusion and ossification of two 

 basidorsal arcualia. Ossificati'on spreads from the neural arch downward into the devel- 

 oping centrum. The centrum, however, develops as a result of perichordal ossification 

 which arises within the membranous connective. tissue around the notochord. The rudi- 

 mentary interdorsals and interventrals probably grow inward into the intercentral spaces 

 to obliterate the notochord between the centra. The interdorsal-interventral complex 

 fuses ultimately with the caudal end of the centrum, to form a rounded knob which 

 articulates with the concave end of the next posterior vertebra. That is, the vertebrae in 

 the frog are procoelous (fig. 321, Q). The urostyie of the frog probably represents a 

 fusion of rudimentary vertebrae caudal to the ninth or sacral vertebra. Vestigial noto- 

 chordal remains may exist in the center of each bony centrum. 



The development of the vertebrae in Necturus (fig. 321, D), resembles that of the 

 frog, with the exception that the bony centrum arises from a perichordal ossification 

 which is entirely independent of the neural arch. Also, the notochord remains continuous, 

 being constricted in the region of the bony centrum, but relatively unconstricted in the 

 area between the centra. That is, the vertebrae are of the amphicoeious type (fig. 321, P). 

 The basiventral arcualia unite to form the hemal arches in the tail. 



III. Development of Vertebrae in the Chick and Mammals. The development of 

 the vertebra in the chick is a complicated affair, as the vertebra is composed of a complex 

 of fused arcualia associated with a perichordal ossification (see fig. 321, F). The vertebrae 

 are heterocoelous, their ends being partly procoelous and opisthocoelous. In mammals 



