FIRST FORMATION OF CRANIAL NERVES. 415 



like the fifth, from the extreme dorsal summit of the neural axis 

 (PI. 15, fig. 4, vn). So far as I have been able to determine, 

 the auditory nerve and the seventh proper possess only a single 

 root common to the two. There is no anterior root for the 

 seventh any more than for the fifth. 



Behind the auditory involution, at a stage subsequent to that 

 in which the fifth and seventh nerves appear, there arise a series 

 of roots from the dorsal summit of the hind-brain, which form 

 the rudiments of the glosso-pharyngeal and vagus nerves. These 

 roots are formed towards the close of stage H, but are still quite 

 short at the beginning of stage I. Their manner of development 

 resembles that of the previously described cranial nerves. The 

 central ends of the roots of the opposite sides are at first in 

 contact with each other, and there is nothing to distinguish the 

 roots of the glosso-pharyngeal and of the vagus nerves from the 

 dorsal roots of spinal nerves. Like the dorsal roots of the spinal 

 nerves, they appear as a series of ventral prolongations of a 

 continuous outgrowth from the brain, which outgrowth is more- 

 over continuous with that for the spinal nerves 1 . The outgrowth 

 of the vagus and glosso-pharyngeal nerves is not continuous 

 with that of the seventh nerve. This is shewn by PI. 15, figs. 4# 

 and 4& The outgrowth of the seventh nerve though present in 

 40 is completely absent in 4^ which represents a section just 

 behind 4^. 



Thus, by the end of stage I, there have appeared the rudi- 

 ments of the 5th, 7th, 8th, Qth and loth cranial nerves, all of 

 which spring from the hind-brain. These nerves all develope 

 precisely as do the posterior roots of the spinal nerves, and it is 

 a remarkable fact tliat hitherto I have failed to find a trace in the 

 brain of a root of any cranial nerve arising from the ventral 

 corner of the brain as do the anterior roots of the spinal nerves*. 



1 In the presence of this continuous outgrowth of the brain from which spring the 

 separate nerve stems of the vagus, may perhaps be found a reconciliation of the 

 apparently conflicting statements of Gbtte and myself with reference to the vagus 

 nerve. Gotte regards the vagus as a single nerve, from its originating as an undivided 

 rudiment ; but it is clear from my researches that, for Elasmobranchs at least, this 

 method of arguing will not hold good, since it would lead to the conclusion that all 

 the spinal nerves were branches of one single nerve, since they too spring as pro- 

 cesses from a continuous outgrowth from the brain ! 



- The conclusion here arrived at with reference to the anterior roots, is opposed 

 to the observations of both Gegenbaur on Hexanchus, Jenaisckc Zeifsc/irift, Vol. VI., 



