236 COMPARATIVE ANATOMY 



to form the stapes of the middle ear, while the ventral portion forms the 

 lesser horn and a part of the body of the hyoid bone. The intermediate 

 portion of the hyoid cartilage forms the stylohyoid ligament by which the 

 hyoid bone is suspended from the petrosal bone of the cranium. The 

 cartilage of the third visceral arch fuses with that of the second, and later 

 ossifies to form the greater horn and part of the body of the hyoid bone. 

 The cartilages of the fourth and fifth arch persist as the thyroid and 

 arytenoid cartilages of the larynx, and also form the cuneiform and 

 corniculate cartilages. The cartilage of the fifth arch is said to form the 

 cricoid cartilage also. Other observers claim that the cartilage of the 

 sixth arch also contributes to the formation of the cricoid. That cartilages 

 of posterior visceral arches form the ring cartilages of the trachea has been 

 asserted on insufficient evidence. 



Just as no theory of special creation can explain why there should be 

 both cartilage and dermal bones in the same human skull, so no theory 

 except evolution can explain why a bone in man should ossify from several 

 centers, each of which corresponds to a separate bone in a lower form. 



n. THE APPENDICULAR SKELETON 



Evolution of Paired Appendages. The cyclostomes have no paired 

 appendages and, so far as the evidence goes, never have had them. We 

 are, consequently, forced to conclude that the paired appendages of 

 vertebrates have no genetic connexion with those of invertebrates, but 

 have arisen independently as vertebrate novelties. Unfortunately for 

 the speculative morphologist, the beginnings of these appendages are 

 obscure. Those of elasmobranchs are the simplest in living vertebrates, 

 but even these are highly differentiated. 



The most promising attempt to solve the problem of the origin of 

 paired fins is the so-called fin-fold theory. According to this, paired fins 

 began as paired folds of skin extending from the region posterior to the 

 gills back to the anus. The paired metapleural folds of amphioxus are 

 often mentioned, with dubious justification, as structures which suggest 

 how the fin-folds may have had their origin. Pectoral and pelvic fins 

 are supposed to be formed by enlarging the end portions of these folds 

 and suppressing the intermediate region. In favor of this hypothesis is 

 the presence of longitudinal paired " Wolfiian " folds in vertebrate embryos, 

 and the fact that the anlagen of the appendages extend through more 

 segments of the embryonic body than do the appendages of the adult, 

 the base of the appendages becoming constricted during ontogenesis. 

 Some morphologists have used the continuous fin-folds of skates as evi- 

 dence supporting the fin-fold theory, while others doubt their significance. 

 The continuity of the folds, as Goodrich has suggested, is probably not an 

 essential element in the theory. See Fig. 193. 



