DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 503 
so evident in Necturus cannot also there be found, for it is 
difficult to believe that the great similarity which exists in the 
position and direction of the main lines of sense-organs in 
Necturus and Acanthias should not be the result of a similar 
course of development. 
Noticing that in Amia the lateral line nerve innervates a 
continuous canal beginning with the sensory differentiation 
above the first vagus cleft, Ewart (loc. cit.) infers that the 
embryonic sense-organ found here gave rise to the pre-com- 
missural, commissural, and trunk portions of the canal, “ with 
or without involving the branchial sense-organs lying above 
the second, third, and fourth vagus clefts,” which probably 
assisted in forming the several vagus ganglia, but have taken 
little or no part in forming the lateral line. 
In Necturus the sensory ridge above the vagus clefts is not 
formed, as are the lateral lines of the trunk, by the prolongation 
of a ridge developing from a given point, but is formed by the 
direct modification of a band of deep ectoderm that lies below 
the ganglion cut from the centre of the primitive epibranchial 
ridge, This sensory line is therefore to be homologised se- 
rially with the sensory differentiations above the hyobranchial 
and hyomandibular clefts, and its anterior extremity is not the 
beginning of the lateral line of the trunk. 
Where, then, are the dorsal nerves which in Amia and Lemar- 
gus should innervate the separate branchial organs above each 
vagus cleft? For, as v. Wijhe (87) tells us, every typical head 
segment should contain on each side, besides its somite, a 
dorsal and a ventral nerve-root. Yes, but it is also true that 
every typical vertebrate segment should include a muscula- 
ture supplied by a motor nerve and a sensitive outer covering 
giving rise to a sensory nerve. The two nerves in a typical 
segment should undoubtedly be connected with that part of the 
central nervous system which their segment includes. 
Consider how far the segments of the vertebrate head have 
departed from their type. 
Beginning with the premandibular segment, we f 
ffelion, 
covered by a sensory epithelium which sends its fibres saditory. 
