192 FIRST FORMATION OF CRANIAL NERVES. 



Behind the auditory involution, at a stage subsequent to 

 tliat 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 moreover continuous with that for the spinal 

 nerves \ The outgrowth of the vagus and glosso-pharyngeal 

 nerves is not continuous with that of the seventh nerve. This 

 is shewn by PL xiv. figs. 4a and 45. The outgrowth of the 

 seventh nerve though present in 4a is completely absent in 

 45 which represents a section just behind 4a. 



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

 rudiments of the 5th, 7th, 8th, 9th and 10th 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 that hitherto I have failed to find a trace 

 in the hrain of a root of any cranial nerve arising from the 

 ventral corner of the hrain as do the anterior roots of the spinal 

 nerves'^. 



1 In the presence of tins continuous outgrowth of the bram from which 

 spring the separate nerve stems of the vagus, may perhaps be found a reconcili- 

 ation of the apparently conflicting statements of Uotte 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 Elasmo- 

 branchs 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, 

 si ace they too spring as processes from a continuous outgrowth from the brain ! 



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

 opposed to the observations of both Gegonbaur on Hexanchus, Jenaische Zeit- 

 scJirift, Vol. VI, and of Jackson and Clarke on Echinorhinus, Journal of Anatomy 

 and Physioloriy, Vol. x. These morphologists identify certain roots sj^ringing 

 from the medulla below and behind the main roots of the vagus as true anterior 

 roots of this nerve. The existence of these roots is not open to question, but 

 without asserting that it is impossible for me to have failed to detect such roots 

 had they been present in the embryo, I think I may maintain if these anterior 

 roots are not present in the embryo, tlieir idontitication as vagus roots must 

 be abandoned ; and they must be regarded as belonging to spinal nerves. This 

 point is more fully spoken of at p. 205. 



