350 Dr. U. Pritchard on the [Mar. 2, 



it; on the outer side of the rods this filament enters the nuclear cell, 

 which has received the descending branch from the middle filament ; and 

 this cell, again, is connected with the first cell of Deiters by another filament. 

 Filaments in the form of festoons connect the hair-cells with each other, 

 and analogous nervous connexions are found between the cells of Deiters. 

 Besides these I have found numerous detached filaments, so arranged as 

 to give the appearance of a plexus throughout the outer part of the organ. 

 This arrangement of the outer group of nerve-filaments has not been 

 demonstrated before; up to the present time it is only the middle 

 filament that has been described. 



Part III. — The Development of the Organ or Corti. 



The organ of Corti is developed from certain of the epithelial cells lining 

 the ductus cochlea?, which at first consist of a single layer of cuboid cells ; 

 later on, those cells which line the floor and sides of the canal elongate 

 into the columnar form, while those lining the inner part of the floor 

 become longer still, and their nuclei multiply two, three, and fourfold. 

 On the outer side of these are five cells, from which the chief part of the 

 organ is developed ; these I term the five primary cells. 



The limbus is next developed and the sulcus formed, the latter being 

 completely filled up by the tall columnar cells with proliferated nuclei y 

 these, however, dwindle down again as age advances. 



The contents of the first or innermost of the five primary cells are 

 divided into two transversely, the upper division forming the inner hair- 

 cell, the lower a nuclear cell. The same change takes place in the third, 

 fourth, and fifth primary cells, their upper division forming the outer hair- 

 cells, and the lower the cells of Deiters. The rods are developed from 

 the second primary cell, which does not divide transversely, but widens at 

 its base so as to form a triangular cell, the inner and outer sides of which 

 thicken and form the rudimentary inner and outer rods. As this widen- 

 ing increases, the protoplasm disappears from the centre, forming the tri- 

 angular tunnel, and the nucleus divides into two, one for each of the lower 

 angles of the tunnel. The rods enlarge at their upper and lower extremi- 

 ties, but do not attain their perfect form until after birth ; at this period 

 the angle of the inner-rod head has not been developed, besides which the 

 shafts are much thicker and more clumsy in form than in adult life. 



The membrana reticularis is developed from the free border of the five 

 primary cells and a few of the other adjacent tall ones, but the exact 

 manner in which the reticulation is formed has not been made out. 



The trabecule are, like the rods, developed from the side of primary 

 cells ; and although only three (on the outer side of the rods) persist in 

 adult life, yet at birth numerous fine trabecule are found to the inner side 

 of the rods. 



I regard the membrana tectoria as a secretion from part of the general 

 epithelial layer ; it first appears as a thick but even layer, and as age ad- 



