SOUND-TRANSMITTING APPARATUS OF CAUDATA 333 



tion, extent, and direction of growth of the isthmus should be 

 noted, in comparison with the columellar fusion w^th the cephalic 

 lips of the fenestra in Amblystoma. Figures 20 and 22 illus- 

 trate the differences. In Amblystoma the columella, which is 

 formed outside the ear capsule, comes to lie against the fenestral 

 membrane, grows toward and secondarily fuses wdth the entire 

 cephalic margin of the fenestra. In this case the columella 

 (fenestral plate) may be said to be the active element in the 

 fusions and the connections looked upon as extraotic tissue. In 

 Spelerpes a narrow region of fenestral lips grows into the fenes- 

 tral membrane, forming the isthmus fenestralis, which differs 

 from this fusion in Amblystoma in three respects: a) in extent 

 and location; h) the fenestral lips represent the active elements, 

 and, c) the tissues forming the connection are strictly otic in 

 source. The two, then, are in no respect alike. Figure 3 rep- 

 resents a section through the apex of the isthmus. Caudad of 

 this level it suddenly decreases in height to the normal level of 

 the fenestral lips. 



In larvae 23 mm. long chondrification has occurred in the 

 columellar cord at the level of the apex of the isthmus. It rests 

 close against the fenestral membrane, but is not included within 

 its tissues. This represents the initial step in the formation of 

 the stylus columellae, which later, through chondrification, ex- 

 tends to the edge of the squamosum. The isthmus, through 

 further growth into the fenestral membrane, reaches the dorsal 

 level of the stylus, and there are indications that a few cells at 

 the growing edge are about to extend further into the mem- 

 brane beyond the level of the stylus and independent of it. The 

 matrix of the tw^o elements comes into contact. An important 

 observation in this respect is that the cells invading the fenestral 

 membrane from the isthmus lie entad of the stylus, and it is 

 due to the growth of the former that the final connection is 

 established. Thus here, as in the formation of the isthmus, the 

 otic tissue is to be considered the aggressive element. At this 

 stage the columellar cord and that portion which has chondri- 

 fied as stylus occupy those relations to the fenestral plate which 

 are maintained throughout life. It never enters more exten- 



