THE SKELETAL SYSTEM 



217 



Goethe, this "vertebral theory" of the skull was developed by Oken in 

 Germany and by Owen in England. The basis of this theory may be 

 seen in any mammalian skull, which consists of four bony rings beginning 

 with the nasal region and ending with the occipital. Owen pictured an 

 archet>'pal vertebrate, the axial skeleton of which consisted of a series of 

 typical vertebrae, the anterior four being enlarged to form the skull. 

 The vertebral theory received its death blow, however, when Huxley 

 called attention to the fact that in the skull of such lower fishes as 

 the elasmobranchs there is nothing remotely resembling a vertebra. The 

 absence of vertebrae where they should be most evident, together with the 

 lack of cranial vertebrae in vertebrate embryos except in the occipital 



Fig. 174. — Ventral (F) and dorsal (D) views of typical chondrocranium showing 

 the cartilage bones derived from it. als, alisphenoid; ch, choana; eo, exoccipital; fl, 

 foramen lacerum; hf, hypophysial fossa; of, orbital fissure; os, orbitosphenoid; qu, 

 quadrate. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



region, led morphologists to abandon the theory. Failure to demonstrate 

 vertebrae in the skull has not, however, altered the opinion that head and 

 trunk had at one time a similar metameric structure. See Fig. 174. 



The notochord forms in chordates the primary skeleton of the head 

 as well as of the trunk. Evidence from comparative anatomy and 

 embryology also indicates that the next step in evolution was the appear- 

 ance of the parachordal and trabecular cartilages. The former as their 

 name suggests parallel the anterior end of the notochord while trabecular 

 cartilages lie anterior to the notochord beneath the forebrain vesicle. 

 Enlarging these cartilages and fusing them with the cartilaginous nasal 

 and otic capsules formed the primordial chondrocranium. The loosely 

 constructed cartilaginous skull of cyclostomes represents roughly this 

 stage of evolutionary development. In the cranium of cyclostomes, 

 however, in addition to the parachordal and trabecular cartilages there is 

 an ethmoid plate anterior to the trabeculae, and the beginnings of a tectum 

 covering the brain in the region between the otic capsules. (Fig. 1 73) 



