15° 



Journal of Comparative Neurology. 



many vertebra like segments as there are gill-arches. The num- 

 ber of vertebae must have been as many as nine, possibly more. 



1st arch. First upper labial cartilage I Ramus secundus 



2d arch. Labial cartilage arch 



3d arch, jaw 



4th arch, hyoid 



5th arch. 1st gill 



6th arch. 2d gill 



7th arch. 3d gill 



8th arch. 4th gill 



9th arch. 5th gill 



I Ramus tertius of 

 J 



I Facial. 



I Glossopharyngeal. 



I Ramus branchialis i 

 V Ramus branchialis 2 

 I Ramus branchialis 3 

 I Ramus branchialis 4 



trigeminus. 



j- Vagus. 



The first to seriously study this question from the embryologi- 

 cal point of view was Balfour. He adopted Gegenbaur's views 

 respecting the post-auditory segments but postulated other 

 primitive nerves which have since disappeared. The facial and 

 auditory are embryologically united, while the former is obvious- 

 ly the hyoid nerve. The trigeminus is similar to the facial, the 

 R. opthalamus profundus being a ramus dorsalis, the R 

 maxillaris inferior corresponds to the main trunk, and the R. 

 maxillaris superior is a ramus praebranchialis ( corresponding to 

 buccalis of the 7th.) 



Subsequently Marshall sought to include the olfactory as a 

 true cranial nerve and homologized the nasal sac with a gill 

 cleft. Marshall also observed the segmentation of the brain tube 

 and attempted to discover a cranial nerve for each expansion of 

 the tube. 



