On the Perilymphatic Spaces of the Amphibian Ear. 249 



Knorpel der äusseren Schale selbst, sondern in deren äusseres (unteres) 

 Perichondrium zu verfolgen." My own observations on both Urodeles 

 and Anura leave me in no doubt as to the derivation of the mem- 

 brane of the fenestra vestibuli from the perichondrium of the outer 

 surface of the chondrocranium. It is therefore comparable with the 

 membrane of the foramen perilymphaticum inferius. The tendency of 

 the perilymphatic spaces to extend themselves beyond the foramina 

 may be seen in the formation of the spatium meningeale and the 

 Saccus perilymphaticus. In all probability the portions of the peri- 

 lymphatic system which are found outside the fenestra vestibuli in 

 both Urodela and Anura are illustrations of the same tendency 

 (figs. 5, 23). 



In Pelobates and E. fusca we saw that the membrane of the 

 Saccus perilymphaticus becomes associated at a certain stage with a 

 structure which had originally no such physiological significance as it 

 then acquires. I should attribute the origin of the operculum (stapes) 

 to a similar "accident". It is not within the scope of this paper to 

 discuss in detail the derivation of the operculum, but there are some 

 considerations arising from my theory as to the origin of the fenestra 

 vestibuli which seem to me to have a bearing on the question. Gaupp 

 says in a recent paper [11]: "Das Operculum der ürodelen ist von 

 den meisten Autoren, die sich mit ihm beschäftigt haben, für einen los- 

 gelösten Teil der Ohrkapsel angesprochen worden." A general agree- 

 ment however, has not been reached, and many investigators regard 

 the operculum as a derivative of the upper part of the hyoid arch. 

 In the absence of any theory as to the mode of origin of the fenestra 

 vestibuli, the description of the operculum as a separated portion of 

 the wall of the capsule cannot be said to advance our understanding 

 of the problem. Proceeding on the assumption that the fenestra vesti- 

 buli has arisen in the manner I have suggested, the operculum may 

 have originated in one of two ways. In the first place, it is possible 

 that it was formed as a chondrification in the central portion of the 

 membrane of the fenestra. Such a strengthening of the membrane in 

 this region might conceivably result in greater physiological efficiency. 

 It is clear that this mode of origin would come nearest to what is 



