252 Journal of Comparative Neurology and Psychology. 



thaeniae) received secondary olfactory tracts and remained at a 

 lower stage of development. 



The absence of epibranchial placodes in segments 2 and 3 

 is noteworthy. If the hypophysis is in segment 2 no placode is 

 to be expected. The absence of one in segment 3 may argue 

 against the supposition that a gill was ever present in that seg- 

 ment. The lense placode and the N. thalamicus would then 

 appear to be the vestiges of the nervous structures connected 

 with the most anterior ancestral gill slit. 



Whether the hypophysis is to be assigned to this segment 

 or to segment i involves the question whether we are to recog- 

 nize a prostomium in vertebrates. The hypophysis invagin- 

 ates about opposite the groove between neuromeres i and ii and 

 extends back beneath neuromere ii. This canal is perhaps 

 equivalent to the buccal cavity of invertebrates. The treatment 

 of the olfactory organ in this paper and the interpretation of 

 the hypophysis as the invertebrate mouth are both consistent 

 with the recognition of the first segmeut as a prostomium. 



d. Segment i. Prostomium. 



Neuromere i, nerve of Locy, olfactory epithelum and nerve. 



The writer is unable to accept Kupffer's idea of a median 

 olfactory organ. It does not appear from his descriptions that 

 the median thickening between the olfactory placodes is ever 

 sensory. Its connection with the brain for some time is to be 

 explained as a mere persistence of the continuity of the neural 

 tube with the ectoderm at the lower edge, of the neuropore. 

 The olfactory sense cells exist from the first in bilateral groups 

 and are innervated by paired nerves. As may be inferred from 

 references to Amphioxus made above, the writer thinks that 

 the interpretation of the olfactory pit as a true olfactory organ 

 requires further evidence. 



The olfactory bulbs are formed by great growth of the 

 lateral walls of the first neuromere. In lower fishes {6j, i 6) 

 and cyclostomes (68) these bulbs still show a structure compar- 

 able to that of the primitive brain wall. The highly speciali zed 

 mitral cells are not primitive features of the olfactory apparatus, 



