Rathke was an able and careful critic of the vertebral theory of the skull, but he accepted it in the main. Actual attack on the theory upon embryological grounds was begun by C. Vogt, in his work on the development of _Coregonus_,[215] and in his paper on the development of _Alytes_.[216] He described for _Coregonus_ an origin of the skull in the main similar to that established by Rathke for the adder. There was a "nuchal plate" in which the front end of the notochord was imbedded; the notochord ended at the level of the labyrinth; there were two lateral bands, comparable to Rathke's lateral trabeculae; a "facial plate" was also formed, which seems on the whole equivalent to the plate formed by the fused anterior ends of the trabeculae. A little later the cranium formed a complete cartilaginous box surrounding the brain, very similar to the adult cranium of a shark. In his criticism of the vertebral theory of the skull, Vogt started by defining the vertebra as a ring formed round the chorda. Now since only the occipital segment of the skull is formed actually round the notochord, the parts of the skull lying in front of this cannot themselves be vertebrae, though they may be considered as prolongations of the occipital or nuchal vertebra. "We must regard the nuchal plate as a true vertebra, modified, it is true, in its formation and development by its particular functions. Now, since the notochord ends with the nuchal plate we can no longer regard as vertebrae the parts of the skull that lie beyond, such as the lateral processes of the cranium and the facial plate, for they have no relation with the notochord" (p. 123). To support this view he adduced the fact that the vertebral divisions (primitive vertebrae) visible in the trunk do not extend into the head. He used precisely the same arguments in his paper on _Alytes_ to destroy the vertebral theory of the skull. We quote the following passage translated by Huxley (1864, p. 295) from this paper. "It has therefore become my distinct persuasion that the occipital vertebra is indeed a true vertebra, but that everything which lies before it is not fashioned upon the vertebrate type at all, and that efforts to interpret it in such a way are vain; that, therefore, if we except that vertebra (occipital) which ends the spinal column anteriorly, there are no cranial vertebrae at all." L. Agassiz, himself a pupil of Doellinger, in the general part (1844) of his _Recherches sur les Poissons fossiles_ (Neuchatel, 1833-43), repeats in the main his pupil Vogt's criticism of the vertebral theory (vol. i., pp. 125-9). These arguments of Vogt and Agassiz were not considered by Mueller to dispose of the theory,[217] which maintained a firm hold even upon embryologists. It was still upheld by Reichert, and Koelliker in 1849 showed himself convinced of its general validity.