NERVUS TERMTNALTS IN AMIA 111 



('09) have shown to be present in the skin of fishes. It is possible 

 that it serves a visceral sensory function although the olfactory 

 pit seems to deve'op as an invagination from the ectoderm. 



The 'ast suggestions receive but little support from the embryo- 

 logical development of the nervus terminalis. Both general cutan- 

 eous and some unspecialized visceral ganglia are developed from 

 neural crest (Landacre, '08 and later unpublished observations), 

 while the epibranchial and dorso-lateral placodes are thought 

 to give rise mainly to ganglion cells of special sensory functions 

 (gustatory and lateral line). The origin of the nervus terminalis 

 from the olfactory placode, therefore, does not support the theory 

 of its general cutaneous function. However, it should be pointed 

 out in this connection that Beard ('88) described neural crest 

 in the developing olfactory nerve and Johnston ('09) has just 

 published an article in which he shows neural crest elements 

 present in lower vertebrates in the region of the neuropore quite 

 near the unpaired olfactory placode. The relation of neural 

 crest with the olfactory placodes in this region has not been 

 worked out fully in the earliest stages of Amia. It is possible 

 that the unpaired olfactory placode (fig. 1) receives neural crest 

 elements that enter the paired placodes to become the ganglion 

 of the nervus terminalis later, but I was not able with my staining 

 methods to differentiate the cells of the nervus terminalis until fully 

 four days after the unpaired olfactory placode had disappeared. 

 Yolk granules (fig. 1) obscure the details in very early stages of 

 Amia and increase the difficulty of tracing possible neural crest 

 elements through four days of embryological history. 



Another possible interpretation of the nervus terminalis has 

 alread}^ been referred to; viz: that its fibers are of sympathetic 

 (visceral) type, probably vaso-motor. The embryological evi- 

 dence here also is obscure for we know of no other case where 

 sympathetic neurones are derived from ectodermal placodes. 

 But if the paired olfactory placodes can be shown to receive neural 

 crest cells, as suggested in the last paragraph, it offers a possible 

 solution of the difficulty; for it is commonly thought that sympa- 

 thetic neurones originate from neural crest (Jones, '05). We 

 have mentioned that Carpenter ('06) found an apparent excep- 



