16 THE ANCESTRY OF VERTEBRATES 



Van Wyhe, and that he considered Gegenbaur's (1871, 

 p. 521) "ventral vagus roots", the hypoglossus of other- 

 authors, (to which we will refer again in the second chapter), 

 as belonging not to the vagus and to the head, but to the 

 spinal nerves. Thus BALFOUR believed that there were 

 no ventral cranial nerves. 



Van Wyhe (1882, p. 40) points to another circumstance 

 which may serve to account for the different character of 

 the dorsal cranial and of the spinal nerves. BELL's (1811) 

 rule of the exclusively sensory character of the dorsal and 

 the motor character of the ventral roots only applies to the 

 striate, voluntary, musculature which is derived from the 

 myotomes, and not for the smooth, visceral, musculature 

 which ows its origin to the lateral plate. While the former is 

 innervated by the ventral spinal nerves, the latter is 

 supplied by the sympathetic nervous system, the ganglia of' 

 which, as shown first by Onody (1885), are separated onto- 

 genetically from the primordial rudiments of the spinal' 

 ganglia, while moreover it has been shown that nerve 

 fibres sometimes pass through the dorsal roots to the 

 visceral muscles, as suggested already by Van WYHE (1882, 

 p. 41). Thus Steinach (1895) demonstrated experimentally 

 that in the frog the dorsal spinal roots contain motor fibres- 

 for the visceral musculature and the bladder. 



In the same way the motor portion of the cranial nerves 

 does not supply muscles derived from myotomes, but the- 

 primordial branchial muscles which, although being striate 

 and voluntary, according to VAN WYHE must be numbered' 

 amongst the visceral muscles, since they are derived from 

 the lateral plate. The difference between the cranial and' 

 the trunk nerves, then, is reduced to the separation of the- 

 sympathetic ganglia from the primary spinal ganglia in the 

 trunk, while in the head this process is absent. Thus in 

 a somewhat different manner we arrive again atBALFOUR's' 

 conclusion that the head in this respect exhibits more 

 primitive features than the trunk, and that, returning to my 

 theory, the dorsal cranial ganglia are more strictly homo- 

 logous to the ventral ganglia of Annelids than are the spinal 

 ganglia, from which the visceral ganglia have separated. 

 Thus according to VAN WYHE the ancestral form of Chor- 

 dates would have had not only dorsal roots of mixed' 

 function but, in addition to the latter, which only innervate 

 the involuntary visceral musculature, ventral roots of purely 



