142 THE EMBRYOLOGICAL CRITERION 



Von Baer, like Rathkc at this time, was also to some 

 extent a believer in the vertebral theory of the skull. In his 

 second volume (1834, pub. 1837) he holds that the develop- 

 ment of the skull, as the sum of the anterior vertebral 

 arches, is in general the same as that of the other neural 

 arches, and is modified only by the great bulk of the brain 

 (Entwickelungsgeschichte t ii., p. 99). He had, however, some 

 doubts as to the entire correctness of the vertebral theory, 

 doubts suggested by a study of the developing skull. " In 

 the course of the formation of the head in the higher animals, 

 something additional is introduced which does not originally 

 belong to the cranial vertebrae. At first we see the vertebra- 

 tion in the hinder region of the skull very clearly. After- 

 wards it becomes suddenly indistinct, as if some new forma- 

 tion overlaid it" (i., p. 194). 



Even more clearly is his doubt expressed in his paper on 

 Cyprinns. " Upon the formation of the vertebral column 

 only this need be said, that at this stage the notochord is very 

 clearly seen, and the upper and lower arches and spinous 

 processes are visible right to the end of the tail, but the 

 separation into vertebrae ceases abruptly where the back 

 passes into the head. I do not hesitate to assert tJiat bony 

 fish, too, have at this stage an unscgiuoitcd cartilaginous 

 cranium (as cartilaginous fish have all their life), the pro- 

 minences and hollows of which constitute its only resem- 

 blance with the vertebral type" (1835, p. 19). 



A convinced supporter of the vertebral theory was 

 Johannes M tiller, who, in his classical memoir on the 

 Myxinoids, 1 discussed at some length the relation between the 

 development of the vertebra and the development of the 

 skull. Mis memoir is principally devoted to comparative 

 anatomy, but in treating of the skeleton he pays much 

 attention to development. He describes the formation of 

 the vertebra: in elasmobranch embryos ; for the facts 

 regarding other Vertebrates he relies largely on work by 

 Rathkc (r>lcnirins, 1833) and Duges (1834). He recognises 

 as the basis of his comparisons the homology of the notochord 



ihcndc An<i/<>mit- dcr M^y.vinoidcn. I'art I. (Osteology and 

 Myology). (Abh. kthiigl. Akatl. U'iss. Berlin, for 1834, pp. 65-340, 9 

 pis., 1836.) Also separate! v. 



