THE HEAD PROBLEM 



609 



the first half of the nineteenth century, the vertebral theory formed an 

 essential part of the orthodox creed of biologists. The brilliant morpholo- 

 gist Johannes Mueller defended it in Germany, while Carl Vogt opposed 

 it. Rathke and Reichert supported it on embryological grounds. In 

 France, Cuvier was coolly indifferent. In America, Louis Agassiz opposed 

 it. 



It was Thomas Huxley who, in 1858, a year before the publication of 

 the Origin of Species, dealt the vertebral theory of the skull its death 

 blow. In his famous Croonian lecture, Huxley pointed out that the fish 

 skull develops primarily as a cartilaginous box, and not as a series of 

 vertebral elements. He thus turned against the vertebral theory embryo- 

 logical evidence which Rathke and Reichert had interpreted in its favor. 

 No vertebra-like structures are present in the cartilaginous skull of 

 elasmobranch fishes, in which, according to hypothesis, vertebrae ought 



NEURAPOPHYSES. 



.NEURAL 



'SPINES. 



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?IBS. "^STERNEBRAE. 



Fig. 503. — A diagram of Owen's archetypal vertebrates based upon the skeleton, 

 modified vertebrae are assumed to form the skull. 



Four 



to be more evident than in higher forms. Although the vertebral theory 

 of the skull never recovered from Huxley's attack, it was Huxley who 

 converted the head problem from a problem of the morphology of the skull 

 only, to one of the structure of the entire head. 



Anatomical Phase. In 1869, Huxley, abandoning the vertebral theory, 

 demonstrated that evidence of head metamerism is found in the series 

 of cranial nerves and the gill-slits with which they are associated. The 

 facial nerve forks over the spiracular cleft, the glossopharyngeus over the 

 first branchial, and the vagus over the more posterior clefts. Huxley 

 assumed that the gill-sHt proper to the trigeminal nerve is the mouth 

 which, in his opinion, might have been formed by the coalescence of a 

 pair of gill-slits. He also suggested that the ophthalmicus profundus nerve 

 represents a pre-oral segment, and that the embryonic orbito-nasal groove 

 to which it is related is the remnant of a gill slit. Huxley recognized 

 altogether nine head segments, four in front of the ear and five behind. 



Two years later, Gegenbaur confirmed the conclusions of Huxley and 

 added the cartilaginous visceral arches as criteria of metamerism. To 

 supply skeletal elements for pre-oral segments, he used the labial cartilages 

 of the elasmobranchs. Gegenbaur, however, erroneously compared 

 visceral arch cartilages with trunk ribs and consequently regarded head 



