STRUCTURE OF SENSIFEROUS ORGANS 317 



gradually acquire the position which they finally 

 occupy. The olfactory organ, therefore, is a 

 specially modified part of the general integu- 

 ment. 



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 sur- 

 rounded by dense and solid bone. Here, however, 

 recourse to the study of development readily un- 

 ravels 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 modi- 

 fications of the adjacent parts. Still more marvel- 

 lous is the history of the development of the organ 

 of vision. In the place of the eye, as in that 



