558 B. r. Kingsbury and H. D. Reed. 



The operculum is formed out of the wall of the ear capsule upon 

 the medial side of the caudal portion of the fenestra. Fig. 23 

 shows the operculum in process of formation. Along the line mark- 

 ing the medial border of the new fenestra the cartilage breaks down 

 and is absorbed, cutting out in this way a large plate from the ventral 

 wall near the caudal end of the ear capsule. In this histolysis, while 

 the end result is a backward and medial extension of the fenestral 

 opening, it is not accomplished by an actual extension back of the 

 fenestra, but the cartilage is absorbed all along the line of separation, 

 several irregular clefts appearing first which afterwards become con- 

 fluent with each other and with the fenestra. At its caudal end the 

 separation of the operculum comes later and at this point a new 

 formation of cartilage occurs extending the operculum in that direc- 

 St.C. 



Fig. 1. Schema illustrating the fusion of the columella with the ear 

 capsule. 



tion outside the ear capsule. New cartilage is, however, formed at 

 the edge of the fenestra as well and in the operculum itself which 

 attains massive dimensions in the fully grown Ambystoma (PI. IV, 

 Fig. 35). 



The inclusion of the cephalic end of the operculum by the lips 

 of the fenestra (PI. I, Fig. 24) is accomplished by a deposit of 

 new cartilage which cements in solidly the fenestral plate and 

 extends the fenestral lip backward outside the operculum. The 

 dorso-cephalic portion of the margin is thus formed by the ventral 

 and caudal edges of the larval columella extended by new cartilage 

 formation, while the ventro-cephalic margin is new cartilage formed 

 upon the ventral lip of the primary fenestra which extends to meet 

 that deposited on the fenestral plate. The diagram, Fig. 1, will 



