No. I.] DEVELOPMENT OE THE EAR OF AMBLYSTOMA. 29 



Pars Basilaris. — The pars basilaris is the last to appear of 

 the diverticula of the sacculus. Hasse ^ failed to find in Siredon 

 pisciformis any trace of it. But Kuhn ^ describes a small pars 

 basilaris in Siredon pisciformis. In the siredon stage of the 

 species that I studied there occurs in the dorsal portion of the 

 lagena a dorsally directed extension of the columnar epithe- 

 lium (Fig. 14), At the time the gills disappear the walls of 

 this extension, originally of uniform thickness, show a differen- 

 tiation into a thickened portion lying next to the lagena, and 

 a membranous portion extending dorso-posteriorly. In the 

 adult the pars basilaris consists of a delicate membranous funnel- 

 shaped extension of the dorsal portion of the lagena closely con- 

 nected with the perilymphatic canal, and of a papilla acustica 

 basilaris situated on the antero-dorsal border of the constriction 

 separating the lagena from the sacculus. The papilla ac. basi- 

 laris, as well as the macula ac. neglecta, possesses a tectorial 

 membrane. 



Rccessiis LabyrintJii. — The recessus represents the last con- 

 nection of the invaginated ear vesicle with the indifferent ecto- 

 derm. The vesicle at the time of its complete closing is pear- 

 shaped and its apical dorsal portion possesses thinner walls than 

 the other parts (Fig. 4). By rapid growth of the lateral wall, 

 this apical portion is pushed mesally and at the same time by 

 dorsally directed growth in itself takes the form of a tube lying 

 close to the brain (Fig. 5). The distal end of the tube becomes 

 enlarged into the saccus endolymphaticus. By farther dorsal 

 growth the recessus reaches the dorsal border of the brain (Fig. 

 12). As already stated, the recessus originally opens into the 

 dorso-lateral portion of the vesicle, but is crowded mesally and 

 ventrally. As the utriculo-saccular partition is developed, the 

 opening of the recessus into the vesicle is situated ventral to 

 this dividing fold. The saccus appears as a distinct enlarge- 

 ment in larvas of 12 mm. in length and grows thereafter with 

 great rapidity, extending dorsally till in the adult the two sacci 



1 Ueber den Ban des Gehororganes von Siredon pisciformis, Anat. Studien, Heft 



4. 1873- 



Die vergleichende Morphologie und Histologie des Gehororganes der Wirbelthiere, 

 Leipzig, 1873. 



2 Ueber das hdutige Labyrinth der Ainphibien, Archiv. f. mikr. Anatomic, Bd. 17, 

 1880. 



