108 CHARLES BROOKOVER 



will be reserved for another paper, but it may be said in this 

 connection that with the. help of Mr. T. S. Jackson, the develop- 

 ment of a ganglionated nerve in two species of Ameiurus has been 

 worked out in detail. The paper will soon be ready for publica- 

 tion and will show that there is the closest similarity in the devel- 

 opment of the olfactory nerve and the nervus terminalis in Ameiu- 

 rus when compared with the account given in this paper for Amia. 



DISCUSSION AND THEORETICAL CONCLUSIONS 



From the embryological history of the nervus terminalis given 

 above for Amia, Lepidosteus, and the teleosts, it is clear that it is 

 to be considered a component of the olfactory nerve rather than 

 a separate segmental cranial nerve. This is in accord with the con- 

 dition which Locy ('99) first described for Acanthias, but later 

 ('05) he came to consider it as arising from a separate placode in 

 the sharks. In the historical sketch I have cited the three points 

 of similarity between the olfactory nerve and the nervus terminalis 

 as pointed out by Pinkus ('05), and I may add here that the gang- 

 lion cells of the nervus terminalis have never been found farther 

 caudad than the sheath cells of the olfactory nerve among which 

 they arise in the fishes that I have studied. 



We will next consider the homology of the nervus terminalis 

 in the fishes. Something remains to be done embryologically on 

 other fishes and there is need of bringing Locy's second account 

 of its development in the shark into agreement with the work 

 on other forms before the homology can be strengthened on 

 the side of embryology, but its adult morphology shows that it 

 is always distributed peripherally to the nasal capsules, that it is 

 in close proximity to the olfactory nerve ventro-mesially and 

 enters the forebrain not far from the neuropore. Also, it has 

 generally been recognized by ganglion cells distributed along 

 its course or more or less aggregated into ganglionic masses. 

 It would be strange if such a nerve were not homologous through- 

 out the fishes. We have quoted Locy ('03) and Pinkus ('05) as 

 thinking so, and Sheldon ('09) is of the same opinion. 



