The Columella Auris in Amphibia. 



609 



Fkog. 

 Two independent elements 

 (a) Pars interna plectri 

 (6) Operculum. 



The plectrum develops in the 

 cephalic, the operculum in the 

 caudal portion of the second- 

 ary fenestra. 



Ambystoma. 



Two independent elements 

 (a) Columella 

 (6) Operculum. 



The columella develops in the cephalic, 

 the operculum in the caudal por- 

 tion of the secondary fenestra. 



3. The operculum chondrifies in the The operculum is cut out of the ear 



opercular tissue and becomes 

 joined to the cartilaginous 

 margin of the fenestra on its 

 dorso-caudal side ; becomes 

 completely separated. 



4. The plectrum chondrifies as an 



independent center out into the 

 dense tissue connecting it to 

 the palatoquadrate. 



5. The plectrum becomes connected 



with the ventral (cephalo-ven- 

 tral) edge of the fenestra. 



6. The M. opercularis inserts upon 



the Operculum. 



7. The cephalic end of the oper- 



culum overlaps the fenestral 

 plate (pseudoperculum) of the 

 plectrum. 



8. The cephalic portion of the sec- 



ondary fenestra closes, exclud- 

 ing the pseudoperculum. 



9. A cephalic extension of the 



cavum perilymphaticum is be- 

 neath the pseudoperculum 

 (Ductus fenestr?e). 



10. The operculum develops first ; the 



plectrum at transformation. 



11. The hyomaudibular nerve passes 



above the plectrum. 



capsule, remaining connected long- 

 est at its caudal end ; becomes com- 

 pletely separated. 



The columella chondrifies as an inde- 

 pendent center in dense tissue con- 

 necting it with the squamosum (and 

 palatoquadrate). 



The columella becomes fused with the 

 cephalic (cephalo-ventral) edge of 

 the fenestra. 



The M. opercularis inserts upon the 

 Operculum. 



The cephalic end of the operculum 

 underlies the columella. 



The cephalic portion of the secondary 

 fenestra closes, including (incorpo- 

 rating) the columella (fenestral 

 plate). 



A caudal extension of the cavum 

 perilymphaticum is beneath the 

 operculum (Recessus perilymphati- 

 cus). 



The columella develops first ; 

 operculum at transformation. 



the 



The hyomaudibular nerve (Vllth) is 

 below the stilus columellse. 



