DERIVATION OF THE LATERAL NERVE. 367 



to the ectoderm, and at the hind end of the body is not yet 

 completely segmented off (abgegliedert) from the ectoderm." 

 Although the last sentence of this quotation may seem to be 

 opposed to my statements, yet it appears to me 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, but only a presumption, in favour of a direct 

 origin of the lateral nerve from the ectoderm ; 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 

 different 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 shewn 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 line, 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 1 , 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 Elasmobranchs arises as a branch of the 

 vagus, and not as a direct product of the external epiblast. 



An interesting feature about the lateral line and the similar 

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

 organs in Elasmobranchs which originate entirely from the 

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

 known facts about the Amphibian epiblast, and the fact that the 



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 Elasrno- 

 branchs are concerned, nearly conclusive against such a derivation of the nerves in the 



head. 



