DEVELOPMENT OF THE EAB. 265 



In lower Vertebrates, this develops into a peculiar cavity 

 filled with calcareous crystals, which in some Primitive 

 Fishes (Selachii} remains permanently open, and opens 

 above on the skull (duct us endolymphaticus). In Mam- 

 mals, on the contrary, the appendage of the labyrinth 

 atrophies. In these, it is of interest only as a rudimentary 

 organ, which has no longer any physiological significance. 

 Its useless remnant traverses the osseous wall of the petrous 

 bone in the form of a narrow canal, and is called the aque- 

 duct of the vestibule (aquceductus vestibuli}. 



Only the inner and lower part (extended like a bladder) 

 of the detached ear-vesicle develops into the differentiated 

 and extremely complex structure which is afterwards known 

 as the 'secondary labyrinth." This vesicle separates at a 

 very early stage into an upper, larger section, and a lower, 

 smaller section. The former gives rise to the ear-pouch 

 (utrieulus) with the three semi-circular canals; from the 

 latter proceeds the ear-sac (sacculus) with the "snail" 

 (cochlea, Fig. 246, c). The three semi-circular canals 

 originate as simple pocket-like processes from the ear- 

 pouch (Fig. 246, E, cse and csp). In the centre of each of 

 these processes, the two walls coalesce, and separate them- 

 selves from the utricle, while their extremities still commu- 

 nicate with its cavity. In all Double-nostrils (Amphirhina) 

 there are three semi-circular canals, as in Man, while of the 

 Cyclostomi the Lampreys have but two, and the Myxinoides 

 but one (p. 103). The highly-developed structure of the 

 "snail" (cochlea}, which is one of the most delicate and 

 admirable products of adaptation in the mammalian body, 

 originally develops very simply as a bottle-like process 

 from the ear-sac (sacculus}. As Hasse has shown, the 



