570 B. F. Kingsbury and H. D. Keed. 



fenestra, later becoming fused with it. There is some evidence in 

 our series that leads us to believe that it will be found to chondrify 

 as a separate piece of cartilage: — (a) its cartilage differs in its 

 staining from that composing the crista semicircularis, presenting 

 the appearance of "younger" cartilage; (b) it lies outside what might 

 be considered the ideal plane of the fenestra and apparently outside 

 the fenestral membrane; (c) in some of the larvae a small cleft is left 

 between the crista semicircularis and the columella. This cleft is 

 especially well marked in the 36 mm. larva (Fig. 48, PI. VII), but 

 present on one side only, being therefore variable in its extent and 

 doubtless simply an indication of imperfect fusion. The columella 

 as in Ambystoma fills in the anterior portion of the primitive fenestra 

 in its dorsal part; if the fusion with the ventral lip occurs early, 

 it is conceivable that the dorsal fusion may be delayed. 



As to the mode of formation of the operculum, the evidence is 

 indubitable; it is forming in the ''floor" of the ear capsule as a 

 chondrification in the "opercular tissue" (membrana fenestrse) in 

 its caudal and medial portions. In the 18 and 20 nma. specimens, 

 the operculum is already outlined, but broadly continuous on its 

 medial and caudal sides with the cartilage of the floor, and were it 

 modelled, would doubtless present the appearance shown in the model 

 of a 20 mm. Triton tseniatus published by Gaupp ('05, p. 695, Fig. 

 350). In the older larvse, save for the larger size of the operculum 

 and its more complete separation from the floor of the otic capsule, 

 conditions have not been essentially changed. A model was made 

 of this region in a 34 mm. larva which is reproduced in Fig. 27 

 (Plate II). Here the operculum is shown forming in the caudal 

 portion of the fenestra still rather broadly connected with the floor 

 of the otic capsule caudally and medially. The rudimentary fenes- 

 tral plate is shown fused with the anterior portion of the crista 

 semicircularis. The closure of the anterior portion of the primary 

 fenestra is also being accomplished by a growth of cartilage upon its 

 ventral side. This figure showing the primitive fenestra and its 

 contained cartilages may be directly compared with the stage in 

 Ambystoma shown in Fig. 24, PL I, and its diagrammatic elucida- 

 tion in Text Fig. 1. 



