COMPARISON BETWEEN THE EAR AND EYE. 185 



The two lamella} of the cup-shaped secondary eye vesicle thus origi- 

 nating, and the mouth of which looks forwards, are continuous with one 

 another at the border of the cup, the outer lamella being continuous 

 with the optic nerve ; the internal lamella alone forms the lining of 

 the cup, and develops into the retina, whilst the external forms the 

 tapetum nigrum. A similar involution is commenced, but is not com- 

 pleted, in the ductus cochlearis. If we conceive such an involution as 

 occurs in the primary eye vesicle to be checked very soon after it had 

 begun, the appearance presented would be that of a vesicle flattened on 

 the side of the involution, and this in section would resemble the sections 

 of the ductus cochlearis marked <? 3 and 4 , fig. 320. The flattened side, 

 or that on which the involution has commenced (corresponding to tym- 

 panal in the figure), would be that which develops into the retina ; the 

 cavity of the vesicle would correspond to that space between the bacillar 

 layer and the pigment epithelium, which at a later period, when the 

 involution has proceeded to its fullest extent, altogether disappears, 

 whilst all the rest of the wall of the vesicle that is not included in the 

 flattened portion would correspond to the flattened or short columnar 

 epithelium of the tapetum nigrum of the choroid. Of course the portion 

 of the wall corresponding to the tapetum is continuous all round with 

 the flattened and thickened portion corresponding to the retina, inas- 

 much as they are both parts of the walls of the same vesicle. The 

 relations of the parts are precisely similar in the ductus cochlearis. Its 

 internal cavity corresponds to the cavity of the primary eye vesicle ; 

 instead of the involution at one spot, we have the innermost layer of 

 its wall (that is to say, the layer originating from the hypoblast, 

 and corresponding to the wall of the primary eye vesicle) undergoing 

 development into a nerve cushion, the auditory nerve apparatus (organ 

 of Corti), which, instead of a disk-like form, has that of a girdle or 

 zone-like lamina, and the innermost layer of cells of which (the hair 

 cells) pass continuously into the remaining epithelial lining of the duct. 

 Even in matters of histological detail the homology between the 

 retina and the organ of Corti still holds good. The epithelium con- 

 tains here, as in the cells of the tapetum nigrum, granular pigment, 

 which, however, is of somewhat lighter colour in the ductus cochlearis, 

 and in Man these pigment granules, as has been already mentioned, are 

 found even in the lamina reticularis. For further details I would refer 

 to the transverse section in fig. 335A. I consider the bacillar layer, as 

 well as the external granule layer of the retina, to be represented by 

 the hair cells in the organ of Corti. The cilia are the analogues of the 

 outer segments of the rods, whilst the protoplasmic bodies of the hair 

 cells are the analogues of the soft internal segments, or rod and cone 



