SOUND-TRANSMITTING APPARATUS OF CAUDATA 339 



newly formed cartilage is pressed against the fenestral mem- 

 brane and, through growth at its periphery, extends over the 

 ectal surface of the membrane and fills the fenestra. The pri- 

 mary fenestra being thus occupied at transformation, the second 

 element or operculum is cut out from the walls of the ear cap- 

 sule, which process forms and fills at one and the same time the 

 secondary fenestra. The stylus in Amblystoma begins as a 

 conical projection from the plate proper and extends toward the 

 squamosum, following the original cord of cells, the unchondri- 

 fied portion of which becomes the ligamentum squamoso-colu- 

 mellare. A comparison of the fenestral structures in Amblystoma 

 and Spelerpes is facilitated if two points are borne in mind: 



1) that the columella of the former is entirely extraotic in origin 

 and, though plate-like, the whole structure is formed through its 

 own growth without the addition of tissue from other sources; 



2) the fenestra is small and almost completely filled in the 

 mature larva by the columella. This influences, without doubt, 

 the method of development of the operculum which is a part of 

 the tissue of the ear capsule cut out into a distinct element. 



In the method of the formation of the plate itself in Spelerpes 

 lies the explanation of the homology of this fenestral element in 

 the Plethodontidae and others to be mentioned later. The 

 primitive cord of cells extending from the ventral edge of the 

 squamosum to the fenestra possesses the same relations to facial 

 nerve and to the arteria carotis and the vena petrosolateralis as 

 in Amblystoma. This structure represents the columella of 

 Amblystoma, but does not spread out into a plate when it 

 comes in contact with the fenestral membrane. It remains a 

 cylindrical rod taking no part in the formation of other struc- 

 tures in the sound-transmitting apparatus. It alone is extraotic 

 in origin, and therefore represents the columella of Amblystoma. 

 During development it becomes attached to the fenestral plate 

 secondarily. The plate itself results from the growth of the 

 isthmus fenestralis into the membrane and is, therefore, com- 

 posed of cells derived from the ear capsule, and hence represents 

 the operculum of the Amblystomidae. 



