40 AYERS. [Vol. VI. 



mental stages of such representatives of Corti's arches are 

 present in the AUigator, and my own studies are only confirm- 

 atory of this view. 



The nerve fibres pass from the cochlear ganglion as medul- 

 lated fibres through the holes of the zona perforata, lose their 

 sheaths, and ascend in the basilar papilla, spreading out fan- 

 shaped in the transverse axis, from the point of entrance and 

 end in the bases of the hair cells. 



Mimns polyglottiis (PI. VI, Figs. 4, 6, 7, 9, and 10; PI. VII, 

 Figs. 1-9 and 12). 



The sense organ of the basilar portion of the cochlea lies 

 upon the basilar membrane as a two (several) layered plate of 

 cells, broadest in its upper portion, gradually narrowing down- 

 wards, like the basilar membrane. It is rounded off at both 

 ends. The sense organ occupies only about two-thirds the 

 breadth of the basilar membrane, but is not confined to this 

 membrane. It rests upon the quadrangular cartilaginous frame 

 of the basilar papilla up to the entrance of the nerve fibres into 

 the organ, or, in terms of mammalian anatomy, the habenula 

 perforata. It is separated from the lagenar sense organ by 

 an interval free of sensory epithelial cells. These two sense 

 organs, though originating by the division of the primitive 

 cochlear sense organ, are in the bird distinctly separated, and 

 their nerves show even more distinctly the separation of the 

 two structures. 



In cross-section of the papilla basilaris the epithelial cells 

 are largest near the inner border, but gradually increase from 

 both sides towards the centre. The highest cells are, however, 

 placed nearly over the edge of the cartilaginous frame. Between 

 the middle and the inner border of the outer third of the basilar 

 membrane the sensory epithelium passes into the ordinary lining 

 of the cochlear canal. The number of cells in transverse rows 

 in the broadest part of the membrane, i.e. near the middle of 

 its length, is about forty. The cells are much shorter than in 

 the other sense organs, and they are well isolated by the sup- 

 porting cells. The cell caps are quite regularly hexagonal. Two 

 important features of difference between the Thrush and Alliga- 

 tor cochlea are the absence in the former of the epithelial ridges 

 found in the alligator and the absence of the larg-e hair cells. 



