230 Journal of Comparative Neurology and Psychology. 



of vertebral arches with the skull and secondary separation of 

 the same ; all these and other changes have been seen in this 

 region by various workers and perhaps all have taken place in 

 different degrees in one or another group of vertebrates. The 

 region as represented is almost the most variable in development 

 and structure that can be imagined. No one criterion can be 

 taken by which to determine the posterior end of the head in 

 vertebrates. The caudal end of the skull varies aiid is of sig- 

 nificance only when interpreted in the light of the nerves and 

 other structures. The use of the first nerve which has both 

 dorsal and ventral roots gives widely differing results, and in no 

 case are we sure that the two roots constitute a complete seg- 

 mental nerve. Myotomes shift, gills are reduced. In truth, 

 the vertebrate head has no posterior limit, although such a limit 

 may be more or less arbitrarily agreed upon for any individual 

 vertebrate species. 



What is needed now in the study of this intricate region is 

 the analysis of its nerves into their components, the discovery 

 of the source of each component and the tracing of its history, 

 and the determination of the segmental position of each nerve 

 and ganglionic anlage with reference to the series of somites. 

 A great number of data are already available for this but nearly 

 all of them must be interpreted in the light of principles which 

 were not appreciated by the original authors. It would not be 

 profitable at this time to attempt to work over this material 

 because there are too few researches in which the myotomes 

 are certainly identified by reference to a standard series such as 

 V. Wijhe's, and because the nerve roots and anlages are not 

 located with reference to the somites. This material has been 

 treated in a comparative manner by Furbringer (35) and his 

 vast work might have cleared up this intricate region if he had 

 understood the relations of the components in the sensory nerve 

 roots. As the writer has pointed out elsewhere (70) he failed 

 to appreciate this. There are indications that the relations in 

 this region are not so complicated and the work of unraveling 

 them need not be so arduous as Furbringer has made it. For 

 Furbringer the dorsal roots of the cranial and spinal nerves are 



