674 THE SKELETAL SYSTEM 



either side represent cartilage bones. (See Table 2.) The hyoid and first 

 branchial-visceral arches form the complicated support for the tongue (con- 

 sult Table 2). 



In mammals, the visceral arches contribute as much to the adult con- 

 dition as in other higher vertebrates. In the human, the caudal portion of 

 the vestigial upper jaw rudiment persists as the incus, and the caudal portion 

 of Meckel's cartilage contributes to the formation of the malleus. The man- 

 dibular arch thus contributes to the important ear bones (fig. 319C-2). The 

 upper portion of the hyoid arch probably forms the stapes; the ventral portion 

 forms one half of the hyoid bone; and the intervening tissue of the primi- 

 tive hyoid arch contributes to the formation of the stylohyal structures (fig. 

 319C, D). The third arch on each side forms the greater horn of the hyoid; 

 the fourth contributes to the thyroid cartilage; the fifth pair forms the arytenoid 

 and cricoid cartilages (fig. 319C and Table 3). 



b. Ossification Centers and the Development of Bony Skulls 

 The formation of the bony crania of all vertebrates entails the use of 

 centers of ossification which involve methods of bone formation previously 

 described. As a rule, one ossification center arises in a single bone, with the 

 exception of those bones, such as the human frontal, sphenoid, or occipital 

 bones, which result from the fusion of two or more bones. In these instances 

 separate centers of ossification are developed in each individual bone. The 

 exact number of ossification centers in all bones has not been exactly 

 determined. 



c. Development of the Axial Skeleton 

 1) Axial Skeleton of the Trunk: a) Notochord. The notochord is one 

 of the basic structural features of the chordate group of animals. It will be 

 recalled (Chapters 9 and 10) that the primitive notochordal band of cells 

 is the physiological instrument which effects much of the early organization 

 of the developing body of the vertebrate embryo. Aside from this basic, ap- 

 parently universal function in vertebrate development, the notochord later 

 functions as a prominent feature in the development of the median skeletal 

 axis. In the cyclostomatous fishes, a persistent, highly developed notochord, 

 enclosed in elastic, and fibrous, connective-tissue sheaths, is found in the adult. 

 The enveloping, connective-tissue sheaths establish a covering for the nerve 

 cord above and for the blood vessels immediately below the notochord. Ver- 

 tebrae are not developed, but in the cyclostomes (Petromyzontia) paired carti- 

 laginous rods lie along either side of the nerve cord above (Goodrich, '30, 

 pp. 27, 28). In the Dipnoi and in the cartilaginous ganoids, such as Acipenser 

 sturio, the notochord persists unconstricted by vertebral elements although 

 supplemented by these structures. In the shark group and in teleost fishes in 

 general, as well as in certain Amphibia, such as Necturus, the notochord 

 is continuous but constricted greatly by the developing vertebral centra. In 



