NERVUS TERMINALIS IN AMIA 59 



cord of cells just described in Amia would seem to correspond in 

 position to the place where von Kupffer's lobus olfactorius impar 

 ('93, fig. 15) comes into contact with his mediane Riechplatte. It 

 was at first suggested to my mind that in Amia the cord of cells 

 might be the ectodermal portion of the hypophysis, but a mass of 

 cells believed to be Rathke's pouch is already present near the 

 optic chiasm. Johnston ('05, p. 202) thinks the lobus olfactorius 

 impar marks the dorsal border of the neuropore. As the first 

 evidences of the nervus terminalis appear much later, no stages 

 of Amia were cut at a date early enough to show the neuropore. 

 Just as this goes to press Johnston ('09) publishes an article 

 describing nerual crest near the olfactory placodes in lower ver- 

 tebrates. The early development of the unpaired olfactory 

 placode and of the neuropore should be investigated fully in 

 Amia. 



At about the middle of their antero-posterior extent the olfac- 

 tory pits come into contact with the brain wall forty hours before 

 hatching, when there is still an unpaired placode. This point of 

 contact is some eight sections posterior to the anterior end of the 

 neural tube, that is to say, about fifty micra. The connection 

 is very slender, as it is comprised within two sections. It can- 

 not be definitely stated that any fibers extend across from the 

 placode to the brain at this time, but there is solution of the 

 limiting membrane of the neural tube at this point. There is a 

 delicate limiting membrane, shown by the fuchsin stain produc- 

 ing a red line, between the mesodermal elements and the caudo- 

 dorsal part of the olfactory placodes, but none could be made out 

 anteriorly and ventrally. The lack of a limiting membrane 

 about the anterior ventral part of the olfactory placodes in the 

 early stages was thought to be due to the recent connection of 

 these placodes with the unpaired placode (fig. 1), and that no 

 membrane has as yet formed in this position. Perhaps this may 

 be the reason why olfactory fibers were observed to appear first 

 at the anterior end of the placode, in most cases, when the olfac- 

 tory nerve develops. 



The condition of the olfactory placodes at a time six hours later 

 than the stage described above, does not differ greatly and has 



