On the Perilymphatic Spaces of tlic Amphibian Ear. 255 



in Urodeles. After metamorphosis it lies within the cavum cranii. The 

 same change in course has no doubt taken place in Rana fusca 

 (temporaria) and Bufo cinereus, but the process in not repeated in the 

 ontogeny of these forms. The (temporary) foramen perilymphaticum 

 accessorium of Gaupp is the necessary result of this alteration in the 

 course of the ductus reuniens. The fusion of this foramen with the 

 anterior end of the fissura metotica has assisted in the forward ex- 

 tension of the latter. 



6. The homology of the foramen perilymphaticum with the fora- 

 men rotundum of higher Vertebrata has not been proved. The same 

 is true of the foramen perilymphaticum inferius of the Anura. There 

 is little doubt however that the foramen rotundum has been derived 

 from one, or both, of these apertures. 



7. In fairly advanced larvae of Rana fusca and Pelobates fuscus, 

 the membrane forming the ventral boundary of the Saccus perilym- 

 phaticus is connected with the wall of the tracheal chamber (Rana), 

 or îung of its own side (Pelobates) by a dense strand consisting chiefly 

 of elongated cells. Since the lungs at this stage are probably functio- 

 nal in part as swimbladders, the arrangement may aid in the trans- 

 lation of variations of hydrostatic pressure into terms of sensation. 

 That the function is concerned with the aquatic mode of life is ren- 

 dered probable by the fact that the strand disappears during meta- 

 morphosis. 



8. The progressive evolution of the pars hasilaris in the Verte- 

 brate series has prohahly been partly the result of its early asso- 

 ciation ivith the perilymphatic system. Even within the limits of the 

 Amphibia, its condition in the higher forms is suggestive of much grea- 

 ter functional importance than is the case in the lower. In evidence 

 of this there is also the fact that in the Anura the perilymphatic re- 

 cessus partis basilaris has acquired direct relationships with the exterior 

 of the skull, thus placing the tympanal area of the pars basilaris in 

 a more favourable position, both for being affected by vibrations, and 

 perhaps also for responding to changes of pressure in the lungs in 

 some larval forms. 



