250 H. S. Harrison, 



presumably meant by development as a separated part of the capsule. 

 If this view is correct, we should anticipate that the operculum would 

 show no signs of an original connection with skeletal parts lying out- 

 side the capsule, that it would develop either internally to the mem- 

 brane or within its substance, and that its thickness would probably be 

 approximately the same as that of the capsular wall. Now although one 

 or other of these anticipations may be realized in the development of 

 various Amphibia, we find, especially in the lower group, the Urodela, 

 that the facts are opposed to this interpretation. Witebsky [12] pointed 

 out that in Siredon pisciformis the operculum is at an early stage con- 

 siderably thicker than the wall of the auditory capsule, and lies on the 

 outside of the membrane of the fenestra. Miss Piatt also [13] states that 

 in Necturus the operculum does not develop as part of the capsule, but 

 arises external to the membrane of the fenestra, between this and the 

 hyoid cartilage. From this observation she concludes that the oper- 

 culum may really be part of the hyoid arch. In my own observations 

 on Siredon pisciformis, 1 have found several features in development, 

 which cause me to incline strongly to the view that the operculum is 

 derived from a skeletal element which had originally nothing to do 

 with the membrane of the fenestra. I have confirmed the above 

 mentioned observations of Witebsky on Siredon. In this form, at 

 15 mm, the operculum lies quite outside the membrane and is not even 

 covered by perichondrium on its external face. Its central portion 

 is also continued outwards as an elevation, which appears to be con- 

 nected with the upper end of the quadrate cartilage by a band of 

 tissue similar to that of which the operculum itself is composed. As 

 I have not definitely occupied myself with a study of the origin of 

 the operculum, I do not wish to lay stress on these observations, ex- 

 cept to emphasize the fact that there is need for a renewed investi- 

 gation of its development in the Urodeles, There are comparatively 

 few observations on these forms, most investigators having drawn their 

 conclusions as to the origin of the operculum from a study of the de- 

 velopment in the Anura, or even in higher forms. 



The manner in which the fenestra vestibuli may have become as- 

 sociated with an element naturally foreign to the capsule may easily 



