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connecting them with the brain. Do we really meet with this condi- 
tion of things in the auditory organ? In other words, is the auditory 
organ merely a specially modified portion of the system of segmental 
sense organs? 
The auditory organ is, like the segmental sense organs, really a 
modified portion of the epiblast. Very early in development it becomes 
shut off in a sac from the epidermis, a condition which only arises later 
in the segmental sense organs. 
The semicircular canals etc. are clearly secondary complications, 
for in every embryo the auditory organ is at first a simple sac shut off 
from the epidermis, of which sac a portion of the inner wall consists 
of two layers of modified epiblastic cells, connected by a dorsal sensory 
branch of a segmental nerve with the brain. 
This double layer of modified epiblastic cells is in every way com- 
parable to a segmental sense organ. As in the latter the cells on the free 
surface possess long hairs. These hairs like those of the segmental sense 
organs are concerned with the perception of wave-motions of the 
medium in which the animal lives. The hairs on the auditory cells 
are indeed concerned with the perception of much finer wave-motions, 
— those of sound, — than those on the cells of the segmental sense 
organs, and hence arises the early shutting off of this organ from the 
skin. The inner layer of cells of the auditory organ is exactly compar- 
able to the inner layer of cells of a segmental sense organ. 
In Teleostei etc. the auditory organ becomes entirely shut off 
from the skin, but in Elasmobranchii the aperture of invagination per- 
sists, and the organ is connected with the surface throughout life, just 
as the segmental sense organs. n 
These facts, together with the fact that the auditory nerve is 
merely a dorsal sensory branch of a segmental nerve seem to point to 
the conclusion that the auditory organ of Vertebratesis fun- 
damentally a specialized portion of the system of sense 
organs of the lateral line, specialized above the rest of the 
system by the acquirement of the more delicate function of the percep- 
tion of waves of sound. & 
In accordance with, and as a direct consequence of, this function 
of receiving waves of sound the auditory organ has been early shut off 
from the external surface, and has developed accessory structures in 
the shape of semicircular canals etc. ‘Thus its primitive simplicity has 
been lost. 
I hope shortly to give elsewhere a more detailed statement of the 
points touched upon in this paper. 
Naples, Dec. 16th 1883. 
