SOUND-TRANSMITTING APPARATUS OF CAUDATA 337 



Amblystoma upon the outer surface of the membrane at a cor- 

 responding stage of development (fig. 7) . Continuous growth of 

 the isthmus and the resulting bars produce the entire definitive 

 plate. Frequently isolated centers of chondrification appear 

 which fuse with other centers and all finally with the plate proper 

 to produce the definitive structure. With regard to the growth 

 of the fenestral plate into the membrane, one observation is 

 important: the advancing edge is thin and the newly formed 

 cartilage is always within the membrane and usually at its very 

 middle (fig. 6). 



The original invading bars of cartilage and isolated centers 

 gradually extend in all directions and eventually meet and fuse. 

 In this way there is produced, in larvae of 43 mm., the con- 

 tinuous plate shown in figure 26 where the chief difference in 

 extent from that in the adult is its failure as yet to fill the whole 

 fenestra. The stylus columellae has the appearance and rela- 

 tions of a rod which is secondarily applied to the surface of the 

 fenestral plate rather than primarily a part of the latter's own 

 substance. Even in the older stages where both elements are 

 ossified, the distinction is evident at certain levels through the 

 loose relations between the two elements (fig. 1). 



The perilymphatic prominence is formed by the outpocketing 

 of the fenestral membrane long before it becomes chondrified, 

 but the M. opercularis is not in evidence until the transformation 

 period arrives. 



Fig. 5 Transection through the fenestra vestibuli of a larval Spelerpes bis- 

 lineatus 31 mm. long. This section is caudad of that in figure 4. A.c, arteria 

 carotis interna; C.I., canalis lateralis ;£'c., ear capsule; /^e.m., fenestral membrane; 

 Op., fenestral plate (operculum) forming within the fenestral membrane; V.p.l., 

 vena petrosolateralis. 



Fig. 6 Transection through the caudal portion of the fenestra vestibuli of 

 a larval Spelerpes bislineatus 31 mm. long. A.c, arteria carotis interna; CI., 

 canalis lateralis; Ec, ear capsule; Fe.m., fenestral membrane; Op., fenestral 

 plate (operculum); V.p.l., vena petrosolateralis. 



Fig. 7 Transection through the fenestra vestibuli of a larval Amblystoma 

 punctatum 35 mm. long. A.c, arteria carotis interna; C.I., canalis lateralis; 

 Col., columella, plate-like and spreading over the outer surface of the fenestral 

 membrane; Ec, ear capsule; Fe.m., fenestral membrane; V.p.l., vena petroso- 

 lateralis. 



