THE SKELETAL SYSTEM 177 



The cartilages of ihe remaining visceral arches in the human embryo 

 have a diversified fate. The dorsal part of the second, the hyoid, ossifies 

 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 aryte- 

 noid cartilages of the larynx, and form also 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 

 contributes to the formation of the cricoid. See Fig. 163. 



II. 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. See Fig. 167. 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 "Wolffian" 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 bases of the appendages becoming constricted dming 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 second step in this evolution was the invasion of connective tissue 

 and muscle into the fin-folds. A similar migration actually occurs in 

 ontogenesis. In elasmobranchs the muscle buds which invade the fin- 

 folds are metamerically arranged. 



The third step in this evolution was the appearance of a series of inter- 

 myotomic cartilage rays like those which support both median and paired 



