146 DERIVATION OF THE LATERAL NERVE. 



probable that Professor Semper has merely seen the lateral 

 nerve partially enclosed in the ectoderm. This position of the 

 nerve no doubt affords a presumption, hut only a presumj^tion, 

 in favour of a direct origin of the lateral nerve from the ecto- 

 derm ; but against this interpretation of it are the following facts : 



(1) That the front part of the lateral line is undoubtedly 

 supplied by branches which arise in the ordinary way from the 

 intestinal branch of the vagus ; and we should not expect to 

 find part of the lateral line supplied by nerves which originate 

 in one way, and the remainder supplied by a nerve having 

 a completely difierent and abnormal mode of origin. 



(2) The growth of the lateral line is quite independent of 

 that of the lateral nerve : the latter arises subsequently to the 

 lateral line, and, so far as is shown by the inconclusive observa- 

 tion of my earliest stage, as an offshoot from the intestinal 

 branch of the vagus; and though it grows along at first in close 

 contact with the lateral nerve, yet it never presents, so far as I 

 have seen, any indubitable indication of becoming split off from 

 this, or of fusing with it. 



(3) The fact that the cranial representatives of the lateral 

 line are supplied with nerves which originate in the normal 

 way\ affords a strong argument in favour of the lateral line 

 receiving an ordinary nerve-supply. 



Considering all these facts, I am led to the conclusion that 

 the lateral nerve in Elasmohranchs arises as a branch of the 

 vagus, and not as a direct p)roduct of the external epiblasf. 



An interesting feature about the lateral line and the similar 

 cephalic structures, is the fact of these being the only sense 

 organs in Elasmohranchs which originate entirely from the 

 mucous layer of the epiblast. This, coupled with the well- 

 known facts about the Amj^hibian epiblast, and the fact that the 

 mucous canals are the only sense-organs which originate subse- 

 quently to the distinct differentiation of the epiblast into mu- 

 cous and horny layers, goes far to prove ^ that the mucous layer 



1 Gotte extends his statements about the lateral nerve to the nerves supplying 

 the mucous canals in the head ; but my observations appear to me, as far as 

 Elasmohranchs are concerned, nearly conclusive against such a derivation of 

 the nerves in the head. 



2 I believe that Gotte, amongst his very numerous valuable remarks in the 

 Eutwichhingsgcschichte der Unke, has put forward a view similar to this, though 

 I cannot put my band on the reference. 



