THE SENSE-ORGANS 485 



there occur in the nasal mucous membrane definite groupings 

 of the olfactory cells, surrounded by protecting cells, thus 

 forming olfactory buds, almost identical in form with the taste- 

 buds, which in turn resemble the lateral line organs. Some have 

 seen in this, as well as in the condition of the primary olfac- 

 tory pits, which, in the embryo, form the anlage of the nasal 

 cavities, a possible genetic connection with the lateral line or- 

 gans, but this homology is rendered improbable from other 

 reasons. Of these the most fundamental is the singular rela- 

 tionship between the individual olfactory, cells and their nerve 

 fibers, the two being directly continuous, and not, as in all 

 other known cases of sensory cells among vertebrates, simply 

 in contact with one another. This continuity of fiber with ter- 

 minal cells is, however, characteristic of the sensory cells of 

 the lower invertebrates and suggests that the sense of smell, 

 or at least the primary olfactory membrane, has been inherited 

 from some far-away invertebrate ancestor, and is thus much 

 older than any of the other sense organs. Another possible 

 relationship with certain other parts will be taken up below in 

 connection with the lens of the eye. 



The essential organ of hearing is the labyrinth or inner ear, 

 a series of membranous tubes or sacs, the complicated struc- 

 ture of which has suggested its name. In fishes it is placed 

 immediately beneath the bones of the head and in its compara- 

 tively superficial position needs no accessory apparatus, but 

 in the higher vertebrates it is located deep in the interior and 

 associates with itself a number of auxiliary parts to aid in the 

 collection and transmission of sound vibrations. Probably the 

 chief reason for this difference lies in the change from water 

 to air, since the denser medium transmits the sound waves with 

 so much more intensity than does the air that the apparatus 

 which develops in adaptation to the former requires an inten- 

 sifying mechanism when placed in the latter. 



The anlage of the labyrinth appears in the early embryo 

 as a slight thickening of the ectoderm over a small lateral area 

 at about the level of the metencephalon. As the cells of this 

 area proliferate more rapidly than those of the surrounding 



