SENSATION AND THE SENSIFEROUS ORGANS 315 



The human ear would seem to present greater 

 difficulties. For the essential part of the sense 

 organ, in this case, is the membranous labyrinth, 

 a bag of complicated form, which lies buried in the 

 depths of the floor of the skull, and is surrounded 

 by dense and solid bone. Here, however, recourse 

 to the study of development readily unravels the 

 mystery. Shortly after the time when the 

 olfactory organ appears, as a depression of the 

 skin on the side of the fore part of the head, the 

 auditory organ appears as a similar depression on 

 the side of its back part. The depression, rapidly 

 deepening, becomes a small pouch; and then, 

 the communication with the exterior becoming 

 shut off, the pouch is converted into -a closed bag, 

 the epithelial lining of which is a part of the 

 general epidermis segregated from the rest. The 

 adjacent tissues, changing first into cartilage and 

 then into bone, enclose the auditory sac in a 

 strong case, in which it undergoes its further 

 metamorphoses ; while the drum, the ear bones, 

 and the external ear, are superadded by no less 

 extraordinary modifications of the adjacent parts. 

 Still more marvellous is the history of the de- 

 velopment of the organ of vision. In the place of 

 the eye, as in that of the nose and that of the 

 ear, the young embryo presents a depression of 

 the general integument; but, in man and the 

 higher animals, this does not give rise to the 



