DEVELOPMENT OF SKULL: HUXLEY 159 



of Meckel's cartilage identical in form and connections in the 

 two groups. The homologies of the bones connected with 

 the jaws in bony fishes had long been a subject of conten- 

 tion among comparative anatomists ; Huxley shows from his 

 personal observations how the development of the visceral 

 arches throws light upon these difficulties. The mandibular 

 arch in the developing fish is abruptly angled, as in the 

 embryo of Tetrapoda ; the upper prong of it ossifies into the 

 palatine and pterygoid ; at the angle is formed the quadrate 

 (jugal, Cuvier), and to the quadrate is articulated the lower 

 jaw, which ossifies round the lower prong or MeckeFs carti- 

 lage. The scheme of development of the jaws is accord- 

 ingly similar in fish to what it is in other Vertebrates, and 

 this similarity of development enables Huxley to recognise 

 what are the true homologues of the quadrate, the palatine 

 and the pterygoid in adult bony fish, and to prove that the 

 symplectic and the metapterygoid (tympanal, Cuvier) are 

 bones peculiar to fish. In developing Amphibia Huxley 

 found a suspensorium of hyoid and mandibular arches similar 

 to the hyomandibular offish. 



Tackling his main problem of the unity of plan of the 

 vertebrate skull, Huxley shows, by a careful discussion of 

 the anatomical relationships of the chief bones in typical 

 examples of all vertebrate classes, that there is on the whole 

 unity of plan as regards the osseous skull. This unity of 

 composition can be established, on the gradation method, by 

 considering the connections of the bones of the skull with one 

 another, their relations to the parts of the brain and to the 

 foramina of the principal cranial nerves. The assistance of 

 the embryological method is, however, necessary in deter- 

 mining many points with regard to the bones developed in 

 relation to the visceral arches. But there is a further step to 

 be taken. "Admitting . . . that a general unity of plan 

 pervades the organisation of the ossified skull, the important 

 fact remains that many vertebrated animals all those fishes, 

 in fact, which are known as ElasinobrancJiii, Marsipobranchii, 

 Pharyngobranchii and Dipnoi have no bony skull at all, 

 at least in the sense in which the words have hitherto been 

 used" (p. 571). The membranous or cartilaginous skull 

 of these fishes shows a general resemblance in its main 



