V. KuPFFER, Cranial Nerves of Vertebrates. 247 



of cranial nerves, sharply distinguishable genetically from 

 the spinal system and belonging to the gill apparatus, a 

 spinal system of dorsal nerves has likewise more or less com- 

 pletely maintained itself. 



The old theory of the spinal nature of the cranial nerves, 

 dating its beginnings from Proschaska and Sommering, re- 

 ceived weighty support through the Goethe-Oken vertebrate 

 theory of the skull; it outlasted the latter doctrine, and ap- 

 peared until recently consistent with Bell's law. But the 

 embryological works of the last decade have so severely 

 shaken the foundations of this doctrine that so decided a de- 

 fender of it as C. Gegenbaur(') found himself obliged to 

 admit that the homodynamy of the cranial and spinal nerves 

 appeared to him no longer tenable. The first shock this the- 

 ory received was through the observation of Balfour,( ') on 

 elasmobranch embryos, that the mixed cranial nerves are de- 

 cidedly of dorsal origin, and they are also to be distinguished 

 essentially from the dorsal roots of spinal nerves owing to 

 the motor elements in them. 



While Balfour did not succeed in finding in the region 

 of the cranial nerves any roots whatever which might be 

 compared with the ventral roots of the spinal nerves, he, 

 on the other hand, did not doubt the complete validity 

 of Bell's law, and accordingly advanced the hypothesis 

 that there had existed originally a common fundamental 

 form of the nerves in the whole body which are only 

 represented by the dorsal roots of a mixed nature. At this 

 stage the diflerentiation of head and trunk took place, so 

 that the type of the spinal nerves corresponding to Bell's law 

 would be regarded as secondarily acquired, while the cranial 

 nerves have preserved the original condition. This hypothe- 

 sis appeared to Balfour so much the more probable in that he 



1 "The Metamerism of the Head and the Vertebrate Theory of the Cranial Skele- 

 ton," Morph. Jahrb , XIII, p. 64 and 104. 



2 "The Development of the Elasmobranch Fishes," Journal Anat. and Phys.,Vol. 

 XI, 1877, and Handbook of Comp Anat , trans, by Vetter, Jena, i88r, Bd. II, p. 411. 



