The Columella Auris in Ampliibia. 559 



illustrate the transformation and the filling in of the primary 

 fenestra and the incorporation of the fenestral plate. The columella^ 

 stilus columellse, and operculum are indicated by Col., 8t. C, and 

 Op. respectively ; the new cartilage is line-shaded, while the region 

 of earliest fusion is cross-hatched. The outline of the included por- 

 tion of the operculum is marked by a broken line. 



In the larva approaching transformation (Fig. 22, PI. I), the 

 operculum is not well outlined, although a slight groove causes a 

 prominence of the wall which later becomes the operculum. As shown 

 in Fig. 31 (PI. Ill), in the sections of the ear-capsule of mature 

 larvae, the region from which the operculum will form, may be 

 recognized. Compare Figs. 31 and 32, in the latter of which the 

 histolysis of the thin cartilage on the medial side of the future oper- 

 culum is severing its continuity with the remainder of the floor of 

 the otic capsule. The operculum thus arises from a portion of the 

 floor of the ear capsule while the secondary fenestra with the plate 

 of the columella is in the lateral wall. (Cf. Figs. 32 and 33.) 



The interior of the ear capsule, opposite the region to become the 

 operculum, is occupied by the cavum perilymphaticum (Figs. 31 

 and 32, PI. III). In the transformed animal this portion of the 

 ear capsule becomes somewhat prominent and may be termed the 

 Perilymphatic Prominence (Prominentia perilymph atica) (Figs. 30 

 and 35). The outward and backward extension of the perilym- 

 phatic space on the inner side of the operculum, as stated above, is 

 termed Recessus perilympliaticus. 



The muscle attaching to the operculum to which Gaupp ('93) 

 gave the name of M. oprrnihris^ extends caudad and at its caudal 

 end is attached to the outer side of the Siiprascapnla. In the larval 

 Ambystoma it is not present. Early in the transformation myo- 

 blasts upon the outer edge of the ventral spinal musculature (M. 



^This muscle in the frog was regarded as a !>t)rtiou of the "Occipito-sub- 

 scapularis" by Duges ('35), the Levator anguli scapulae of Eeker. Cope 

 ('88 a and b) has termed it M. stapedius, Iwanzoff, M. protrahens scapulte. 

 Gaupp subseciuently ('96) designated it the "Pars opercularis of the M. Leva- 

 tor scapulfe superior." In Urodela it seems to deserve recognition as a 

 distinct muscle, and its development apparently differs somewhat in the two 

 groups. 



