432 C. W. PRENTISS 



the spiral organ (fig. 13). The cochlear duct of the pig makes 

 about 3.5 turns and hence a section through the modiolus shows 

 four turns on the left and three on the right in figure 13. Figure 

 2 represents the basal half of the turn lettered (2) in figure 13. 

 Of the two groups of cells which we have noted above the larger 

 group forms the inner (axial) two-thirds of the basal wall. The 

 nuclei of these cells are arranged in from three to six layers. 

 They are separated from the outer group of cells by a cytoplasmic 

 area free from nuclei. Between two cells of this area a vertical 

 space is seen extending from the summits of the cells half way 

 through the epithelium. This space may be due to shrinkage 

 but represents the position of the spiral tunnel or tunnel of Corti 

 in later stages. The outer cell group (lesser thickening of Ricken- 

 bacher) forms the outer third of the basal wall of the cochlear 

 duct. It will eventually give rise to that part of the spiral organ 

 which lies external to the spiral tunnel (fig. 1). From the inner 

 epithelial cell group (greater epithelial thickening) will develop 

 the epithelium of the labium vestibulare, of the internal spiral 

 sulcus and that portion of the spiral organ lying internal (axial) 

 to the tunnel. Extending over the free ends of the cells of the 

 greater epithelial thickening may be seen a cuticular membrane 

 which is attached between the cells by delicate threads. This 

 cuticular membrane is the anlage of the membrana tectoria, 

 which thus makes its appearance before the hair cells of the 

 spiral organ are differentiated. At this stage the mesenchyma 

 about the cochlear duct is dense and the scalae have not yet 

 appeared. The nerve fibers of the spiral ganglion may be seen 

 entering the epithelium internal to the organ of Corti. 



8.5 cm. stage. In the second turn of the cochlea at this stage 

 (fig. 3 and fig. 13)- a space representing the spiral tunnel extends 

 nearly through the thickness of the epithelium. The cells on 

 each side of the tunnel are differentiating the pillars of Corti 

 and a single inner and three outer hair cells are conspicuous. By 

 the rapid division and elongation of the cells of the greater epi- 

 thelial thickening the epithelial wall has been bent basalwards 

 forming a concavity above and a convexity below. The concavity 

 is the first trace of the internal spiral sulcus. The nuclei of the 



