330 H. D. REED 



In the caudal portion of the plate is a perilymphatic promi- 

 nence, in a depression of which the M, opercularis is attached 

 (fig. 22). 



From the preceding paragraph it is obvious that Spelerpes 

 and other genera have a structure which combines the charac- 

 teristics of both columella and operculum of the amblystomid 

 type. The natural inference is that in these forms the single 

 fenestral element represents some sort of fusion of columella and 

 operculum. With limitations, this statement is true, but the 

 significance of 'fusion' in this relevancy becomes evident only 

 after the examination of numerous developmental stages. 

 Structures which are here considered to be the homologues of 

 the columella and operculum are present, as was mentioned 

 briefly in an earlier communication (Reed, '14) and in the nature 

 and order of appearance of these structures in Spelerpes there is 

 identity with Amblystoma; that is, the columellar representa- 

 tive appears first, followed by an element of otic derivation which 

 may be compared with the operculum. 



The proton of the columella appears first as a cord of cells 

 extending from the squamosum to the region of the fenestra, 

 while the plate filling this opening appears later. The develop- 

 ment of these structures in Spelerpes is much retarded as com- 

 pared with Amblystoma. In the latter, embryos 4 mm. long 

 possess a well-defined columellar cord in its typical position and 

 relations, while in Spelerpes bislineatus embryos 8 mm. long 

 this whole region is filled with undifferentiated mesenchyme. In 

 embryos 10 mm. in length the columella is discernible as a dense 

 cord which bears a significant relation to the hyomandibular 

 cleft which is seen as a very pronounced continuation of the 

 oral epithelium reaching beyond the ventral border of the ear 

 capsule (fig. 2), but the double folds do not separate to form a 

 cavity. Just above the dorsal end of this cleft the proton of the 

 columella may be seen as a dense mass of cells which, farther 

 caudad, extends ventrad toward the fenestra, where it ends 

 abruptly. In all of its relations it occupies the typical position 

 above the facial nerve and between the artery and vein of this 

 region. The columella at this stage is a dense segregation of 



