62 THE ANCESTRY OF VERTEBRATES 



throw a new light on the old question of the metamerism 

 of the Vertebrate head, which perhaps will prove able to 

 contribute as much to the disentanglement of this compli- 

 cated problem as the numerous anatomical and embryolo- 

 gical researches to which it has given rise, thus demonstrating 

 once more, that theory and practical research are comple- 

 mentary to each other. Ii is not my intention to give an 

 historical enumeration of all the investigations and different 

 opinions on this subject, the less so, as I can refer to the 

 excellent reviews of RABL (1892) and Gaupp (1898, 1^06). 

 The mam points may, however, be summarized. HuXLEY 

 (1858), after having demonstrated the inadequacy of GOETHE's 

 and Oken's vertebral theory of the bony cranium, accor-. 

 ding to which the latter is only the forward continuation 

 of the Vertebral column round the brain, was the first to 

 try to analyze the metameric structure of the head by 

 studying the mutual relations of gill clefts and cranial nerves. 

 Equally on grounds borrowed from comparative anatomy 

 were based the conclusions of GegenbauR (1871, 1872), 

 who was the first to treat the subject from a phylogenetic 

 .point of view. He transferred the starting point of the 

 problem from the bony to the cartilaginous head skeleton, 

 as found permanently in such primitive Craniates as the 

 Elasmobranchs, and thus inaugurated a new era in the 

 researches concerning the inetameric structure of the head. 

 GEGENBAUR set out from a comparison of the visceral archs of 

 the head to the inferior archs and the ribs of the trunk skeleton, 

 both being the expression of a corresponding metameric 

 structure. In the cranium itself, truly, this metameric structure, 

 is less evident, an intimate concrescence of the verte- 

 brae having occurred at the anterior end of the body, to 

 provide a firm support for the snout and for the insertion 

 of the visceral muscles, while the incorporation of the 

 auditory capsule in the wall of the cranium has contri- 

 buted to efface the dividing lines of the vertebrae. Traces of 

 former segmentation, however, were found by GEGENBAUR 

 in the relation of the cranial nerves to the gill-slits. 

 Taking into consideration the extension of the notochord 

 into the base of the cranium, and emphasizing, as HuXLEY 

 had done already, the contrast between the N. olfactorius and 

 opticus on the one side and the remaining cranial nerves on 

 the other, GEGENBAUR distinguished a posterior vertebral 

 from an anterior prae-or evertebral part, of which 



