No. I.] 



THE VERTEBRATE EAR. 



209 



recalls the entirely similar method of development, described by 

 Villy, for the Frog, a member of the Ichthyopsida. Here, how- 

 ever, the canal itself does not develop until after the anterior 

 and horizontal canals have made their appearance, so that the 

 conditions of development are even more modified. 



When judged on the basis of the adult conditions found in 

 the members of the mammalian group, the ontogenetic develop- 

 ment of the cochlear tube progresses much more rapidly than 

 the differentiation of the sense organs contained. Phylogenet- 

 ically the organ of Corti is well developed before the tube has 

 made its first spiral turn ; i.e. while still in the Sauropsidan con- 

 dition, as exemplified in Echidna and Ornithorhynchus, but 

 ontogenetically this is not the case ; for in the ears of the 

 embryos of Man, Ox, Pig, Rabbit, Mouse, and Cat, the organ 

 has scarcely begun its histological differentiation at the time 

 when the adult condition, as regards number and form of the 

 spiral turns of the cochlear 

 tube, has been attained. 



Mainly on account of the 

 great development of the brain 

 in the human species more 

 than in any other Mammal, 

 and the consequent altered 

 relations of the cranial bones, 

 the vertical axis of the lower 

 vertebrate ear is shoved to 

 one side, so that in Man, for 

 example, it is directed from 

 within, below, and in front — 

 outwards, upwards, and back- 

 wards. In keeping with this 

 change is the alteration in the 

 direction of the horizontal axis 

 of the lower vertebrate ear, so 

 that in Mammals it comes to 

 nearly coincide with the verti- 

 cal axis ; in other words, there 

 has been a revolution of the ear on two of its axes, the main 

 change having taken place by a revolution of nearly 90° in the 

 plane of the vertical axis. 



cr.o.poat- 





Cut 24. 



■See explanation of PI. IX, 

 Fig. 3- 



