248 
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 reseaich are comple- 
mentary to each other. It is not my intention to give an 
historical enumeration of all the investigations and different 
opinions on this subject. the less so, as [ can refer to the 
excellent reviews of RABL (1892) and GAUPP (1898, 1506). 
The main 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 metameric structure of the head. 
GEGENBAUR set out from a ccmparison of the visc: ral 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 contrí- 
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 
