AYERS. 



[Vol. VI. 



von grosserem Umfang als die spatere Grube, und die Gran- 

 zen des Pleckens verlieren sich nach alien Seiten bin in ziem- 

 lich unbestimmter Weise." Balfour's observations do not go 

 further than this, so that we must rely on the adult anatomy for 

 our conclusions as to the probable course of development of 

 the integral parts of the bird's ear. The structure of the adult 

 internal ear of birds is so very similar to that of the reptilian 

 type that there can be little doubt of the essential agreement 

 of the phases of its development with those of the reptilian ear 

 given above. 



Development of the Ear in Mammals other than Man. 



Krause, who has recently studied the development of the 

 mammalian internal ear, finds that in the Pig and Sheep, when 



(xmt 



Cut 21. — The ear of a very 

 young Rabbit embryo, 1 1 mm. 

 long. Figure after Krause. 



a Anterior canal. 



ant Ampulla. 



d Ductus endolymphaticus. 



/ Lagena. 



J Sacculus. 



u Utriculus. 



Cut 22. — The internal ear of a some- 

 what older Pig embryo, t^t, mm. long. 

 Figure after Krause. 



fully cut off from the exterior (and this occurs in embryos 6 mm. 

 long), the auditory vesicle forms a compressed sac, having an 

 irregular quadrangular shape. The utricular and saccular divis- 

 ions are readily distinguished, and already the semicircular 

 canals have begun to differentiate by the formation of two 

 shallow depressions in the wall of the utriculus (of authors) 

 (Cut 2i) ; both the pocket common to the vertical canals and 



