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actly the same way as a typical section of the lateral sensory canals. 

 If the lips of this pit do not coalesce, the adult fish has a single nasal 

 aperture, while, if they coalesce, two apertures are formed, and these 

 two apertures are capable of travelling away from each other a con- 

 siderable distance along lines predetermined by certain conditions, 

 whatever those conditions may be. One of the two apertures so- 

 formed lies nearer the mouth than the other, and may be called the 

 anterior nasal aperture, the other one being the posterior aperture. 

 In Teleosts and the bony Ganoids both of these apertures lie on the 

 outer surface of the head, wholly external to the mouth cavity, though 

 one or both of them, I am not sure from the description which, may, 

 according to Sagemehl (No. 36, p. 223), be found in the upper lip 

 in certain Teleosts. In Elasmobranchs the olfactory pit becomes an 

 unclosed groove, and the anterior end of this groove, prolonged by 

 the so-called naso-labial groove, may come to lie at the upper edge 

 of the mouth opening, against or close to the external surface of the 

 pterygopalatine line of teeth. In Dipnoids this anterior end of the 

 nasal groove becomes enclosed in the labial portion of the mouth 

 cavity by the formation of an upper lip beyond it, the open nasal 

 groove having been previously closed to form a canal. In amphibians, 

 and certainly in certain of the higher vertebrates, the anterior aperture 

 becomes wholly enclosed in the mouth cavity, the nasal groove, as it 

 extends orally into the mouth, passing, in all descriptions I can find of 

 it, between the "Anlagen" of the mammalian premaxillary and maxillary 

 bones. This method of development of the internal nostril, known to 

 take place in certain of the vertebrates above Pisces, I personally 

 believe to be, in principle, the invariable method, Hochstetter's 

 assertions (No. 17 and 18) to the contrary notwithstanding. This I 

 have already had occasion to state in an earlier work (No. 3, p. 460), 

 and since then Brauer (No. 7) has shown that the internal nares are 

 thus developed in the Gymnophionae, and Seydel (No. 42) that they 

 are similarly developed in Echidna, Apparent departures from this me- 

 thod of development are, I believe, simply abbreviations of the process. 



Opinions have differed as to which of the two apertures of Te- 

 leosts is the homologue of the internal nasal aperture of the higher 

 vertebrates, but I believe the weight of opinion is in favor of the 

 views given above. 



If now the internal nasal apertures of vertebrates higher than 

 fishes are formed as stated above, and a maxillary breathing valve, 

 or the bones related to it, are present in those higher animals, it is 

 evident that the internal apertures, in gradually acquiring their final 



