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the vagus supplies six posterior branchial clefts, and must therefore be 
equivalent to at least six segmental nerves (Marshall). As a natural 
result of this partial fusion of the main roots of the original nerves, a 
reduction of the number of dorsal branches to the segmental sense or- 
gans, due in turn to the fusion of these branches, takes place. 
-- Van Wijhe finds three dorsal branches in Scylliwm and Pristi- 
urus, one for each root of the vagus. The first of these, Ram. supra- 
temporalis, which'also forms a separate »lateral nerve« in the adult 
ventsteht durch die Verschmelzung des Ram. I: vagi mit einer isolirten 
Epidermisverdickung, welche später sowohl. einen Theil des Haupt- 
schleimcanales als der supratemporalen Quercommissur zu bilden 
scheinte (Van Wijhe). According to the same author the main la- 
teral nerve arises as a 2 nerve ao Lo and and ard ramus of 
the vagus. RCE 
In Teleostei this fusto er gone ratio fuktheh; there is sal one 
lateral nerve given off by the vagus. Sa 
But as seen above in Elasmobranchii traces of a primitive condition 
of the lateral nerve exist, —-of a condition in which there was for each 
root of the vagus a dorsal ‘branch: die i sense SERE BE its 
segment. 
From this point to the end of the body no traces are to be found 
of segmental branches to the segmental sense organs, and what we 
have instead is the remarkable phenomenon of a sensory branch of a 
cranial nerve fusion (the vagus) supplying fibres to sense organs which 
extend to the extreme posterior end of the body. How did this con- 
dition of things arise? The sense organs of the lateral line in the body 
are just as segmentally arranged as those in the head. The only differ- 
ence obtains in the nerve supply, for in the head the nerve of each 
segment gives off a dorsal branch to supply its corresponding pair of 
sense organs, in En body La: is a one nerve stem to all the sense 
organs. 
The ona of the condition of things i is to be looked for in 
the partial fusion of the segmental nerves which form the vagus. 
Through this fusion, which ‘as shown above takes place from behind 
forwards, a corresponding fusion of the dorsal sensory branches of the 
constituent nerves took place, and thus a lateral nerve or more than 
one was formed, which innervated more than one pair of sense organs. 
Hence the vagus complex dita a Brewer share in he: innervation 
of the lateral line. ita ag 
The dorsal branches of Rent nerves in the pally in GRA 
with Dohrn’s principle of change of function, gradually lost their 
function of innervating segmental sense organs , and this function was 
