No. I.] THE VERTEBRATE EAR. 205 



3. In the Rabbit One Week Old. 



The large epithelial ridge is disappearing in the apical turn, 

 and it is much reduced elsewhere. The membrana tectoria lies 

 only on the crown of the large ridge, and a space has appeared 

 between it and the epithelial surface which becomes the sulcus 

 spiralis intern us. 



The pillar cells have developed their pillars and separated 

 from each other, forming the tunnel, except at the very apex 

 of the cochlea. Nuel's space is now more fully developed than 

 at any previous time, and is relatively as well developed as in 

 the adult. Hensen's supporting cells have developed and they 

 form the highest part of the small ridge, which appears much 

 larger than the now much reduced large ridge. Hensen's sup- 

 porting cells are best developed in the middle turn at this stage. 



The membrana tectoria reaches outward to a point above the 

 outer hair cells, and its outer zone has grown thicker. In the 

 apical turn it hardly reaches to the second row, whereas its 

 fibrous processes reach downwards to the Deiters cells, as 

 before described. In the middle and basal turns these fibres 

 are broken off, and one finds only remnants preserved in their 

 attachment to the hair cells. 



4. In the Ten Days Embryo. 



The organ of Corti is now nearly complete. All that remains 

 of the large epithelial ridge is the inner supporting cells, and 

 they are lower now than those of the small ridge, which forms 

 the highest part of the papilla acustica basilaris. The pillar 

 cells are almost fully developed. The tunnel of Corti and 

 Nuel's spaces are fully developed, as are also the inner and 

 outer hair cells, and they have taken their adult positions. 

 Hensen's body is to be seen near the upper end of the outer 

 hair cells. Deiters's cells have grown much longer and carried 

 the nerve end cells upward. The membrana tectoria reaches 

 to the outer hair cell row and shows traces of the broken fixa- 

 tion fibres present in earlier condition, partly on the upper ends 

 of the third row of Deiters's cells, and partly on the border of 

 the membrane itself. The vas spirale is recognizable by the 

 reduced rudiments of its walls. 



By the fourteenth day the cochlea of the young Rabbit has 



