SOUND-TRANSMITTING APPARATUS OF GAUD ATA 349 



■formation of a matrix. In larvae 27 mm. long the stylus is 

 chondrified for nearly its whole length and is loosely joined to the 

 cartilage of the rapidly forming plate. 



The origin of the elements in Desmognathus is identical with 

 those of the Spelerpes group, but the role of the isthmus is not 

 so important. Figure 30 illustrates the mode of development. 

 The isthmus, instead of extending directly upward and sending 

 out both dorsal and caudal arms, thus effecting a union between 

 it and the stylus, sends out only the caudal arm which grows 

 diagonally upward and backward, never coming in contact with 

 the stylus at all. Sooner or later it meets isolated centers of 

 cartilage with which it fuses, and these centers in turn fuse with 

 each other, thus establishing a band of cartilage between the orig- 

 inal isthmus and the independently formed stylus. This band, 

 when finally completed by the fusion of isthmus and isolated 

 centers, extends to the caudal margin of the fenestra, where it 

 bends upon itself, sending a bar of cartilage cephalad to join the 

 end of the stylus as mentioned above. The place of fusion is 

 marked F in figure 30. The fenestral bar is well outlined in 

 larvae 26 mm. long, but an unchondrified space exists between 

 it and the columellar cord of cells. Gradually these spaces are 

 filled by growth, but certain areas remain open until filled by 

 bony tissue. 



By way of summary, it may be stated that in Desmognathus 

 the plan of development and morphology of parts conform strictly 



Fig. 11 Transection through the fenestral plate and stylus of an adult Des- 

 mognathus fusca. Col., stylus columellae (columella); Op., operculum (fenestral 

 plate). 



Fig.l2 Transection through the fenestra vestibuli of a larval Desmognathus 

 fusca 26 mm. long. Col., columellar proton; Ec, ear capsule; Fe.m., fenestral 

 membrane; Sq., squamosum. 



Fig. 13 Transection through the middle of the fenestra of a larval Desmog- 

 nathus fusca 26 mm. long. Ec, ear capsule; Fe.m., fenestral membrane; /./., 

 isthmus fenestralis; Op., isolated cartilage cells which through growth and fusion 

 with others form the fenestral plate or operculum. 



Fig. 14 Transection through the fenestra of a Desmognathus fusca larva at 

 a level slightly caudad of that of figure 13. Length of specimen, 26 mm. Ec, 

 ear capsule; Fe.m., fenestral membrane; Op., fenestral plate (operculum) formed 

 by the fusion of isolated areas of chondrification. 



