THE SKELETAL SYSTEM 



159 



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

 beginning with the nasal region and ending with the occipital. Owen pic- 

 tured an archetypal 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 lishes as 

 the elasmxobranchs, 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. 149. — Diagrams showing the development of the primordial skull. Since this 

 organ develops primarily beneath the brain as a support the figures represent the ventral 

 aspect. (A) Early stage, before the appearance of cartilage. The notochord is seen 

 lying along the nerve cord as far forward as the hypophysis. The three sense-organs, 

 nose, eye, and ear, have already appeared. (5) This stage shows the trabeculae (0. 

 the parachordals (p), and the capsules around the sense-organs. (C) In this the 

 trabeculae, the parachordals, and the nasal and otic capsules have fused into a single 

 mass, the primordial skull, or chondrocranium. The anterior end of the notochord is 

 imbedded in this. The cartilaginous capsule of the eye remains free to allow the 

 necessary movements of the eyeball. (From Wilder's "History of the Human Body," 

 Courtesy of Henry Holt and Co.) 



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. 



The notochord forms in chordates the primary skeleton of the head 

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

 embryology 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 



