240 HOWARD AVERS. [Vol. IV. 



consider an organ unpaired and median when its essential 

 structures are distinctly paired and bilaterally symmetrical, — 

 even though some accessory portion has been so modified as to 

 have lost all trace of its double origin. So far as I have dis- 

 covered, the general acceptation has been that the Cyclostomes 

 possessed only a single nasal organ, a single median olfactive 

 area to which both of the olfactory nerves gave up their fibres. 

 It is true, this supposed relation of two olfactory nerves to a 

 single olfactive area has caused comment, and the explanation has 

 been offered that we had to do with a modified condition, — a 

 degradation of high sense organs. But as my sections show, 

 the proximal portion of the nasal pit is divided by a median, non- 

 olfactive raphe, into two lateral pockets, or right and left nasal 

 pits, to which alone the olfactory nerve of the right and left 

 sides, respectively, are distributed. The median territory is sup- 

 plied by branches from another nerve, which one I have not 

 been able to determine specifically. 



I wish to call attention to the fact that, among the lower 

 vertebrates there is a connection of the two olfactory pits across 

 the median line by way of the mouth, here especially distinct, but 

 among the higher forms it is also evident. This connection is 

 usually brought about by means of grooves, placing the olfactive 

 pit in communication with the mouth. Even in the mammalia 

 the formation of a median partition to the extent of the forma- 

 tion of two nasal canals is a secondary process. The primitive 

 or embryonic condition shows two simple pits, separated by a 

 more or less distinct partition, which is usually a broad one. 

 Of course in this stage the pits are not deep, and by the time 

 their boundary walls become extensive the median portion is 

 also well developed. In Petromyzon the partition remains 

 rudimentary, 



I have, from my studies on Ammocortes and Petromyzon, 

 ventured to make my statement a general one for the Cyclosto- 

 tomata ; and I feel sure there is no reason to doubt that Myxine 

 and Bdellostoma will be found to have their nasal apparatus, 

 bilaterally symmetrical, with as great distinctness as Petromyzon 

 shows this very important condition. The embryonic rudiment 

 of the nasal pits in the earliest stages of epithelial differentia- 

 tion, needs further study, as none of my stages are young 

 enough to prove that the olfactive areas arise entirely independ- 



