BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 173 



In the first place, I think it will be of great advantage and will tend 

 to simplify matters very much if the general schema of the develop- 

 ment of a cranial nerve (dorsal root) of an Elasmobranch, such as 

 Torpedo, be given. 



Then those cranial nerves, which I regard as segmental, will be dis- 

 cussed : olfactory, nerve of ciliary segment, trigeminal, facial, auditory, 

 glossopharyngeal, and vagus. 



The optic nerve is left entirely out of consideration. Firstly, 

 because I have made no investigations, and hence have no new facts 

 about it to record ; and secondly, as is well known, its whole develop- 

 ment is different from that of the other cranial nerves ; and I can only 

 agree with those zoologists who class the optic nerve entirely apart 

 from the other cranial nerves. 



Not so, however, with the olfactory and auditory nerves and organs. 

 Partly following Marshall, I feel bound to place these nerves in the 

 category of cranial segmental nerves, and to class the olfactory and 

 auditory organs 1 as specialised branchial sense organs. 



Finally, after the account of the various nerves, the bearing of the 

 facts described on the morphology and ancestral history of Vertebrates 

 will be discussed. 



General Schema op the Development op a Dorsal Eoot of a 

 Cranial Nerve. 

 According to the existing views of the development of a dorsal root 

 of a cranial nerve in Elasmobranchii, based mainly on the researches 

 of Balfour, 2 Marshall, 3 and Van Wijhe, 4 the nerve, soon after its 

 development from the neural ridge, divides into two main branches, a 

 dorsal one and a ventral one. The dorsal branch is sensory, and 

 supplies the so-called organs of the lateral line. The ventral one is 

 mainly motor ; it soon divides again into two branches, which, as 

 Stannius 5 first showed, pass one on each side of a visceral cleft. The 

 posterior branch is mainly concerned with the innervation of the 



1 Beard, "On the Segmental Sense Organs, &c," 'Zool. Anzeiger,' 161, 162, 18S4 ; "On 

 Cranial Ganglia, &c," 'Zool. Anzeiger,' 192, 1885. 



2 Balfour, " Elasmobranch Fishes." 



3 Marshall, "The Development of the Cranial Nerves in the Chick," ' Quart. Journ. Micr. 

 Sc.,'1878; Marshall, " On the Head Cavities, &c," 'Quart. Journ. Micr. Sc.,' 1880; Marshall, 

 " On the Segmental Value of the Cranial Nerves, &c," ' Journ. of Anat. and Physiol.,' 1882 ; 

 also separate. 



1 Van Wijhe, *Ueber die Mesodermsegmente und die Entwickelung der Nerven des 

 Selachierkopfes,' Amsterdam, 1882. 



' Stannius, ' Das Peripherische Nervensystem der Fische,' 1819. 



