No. I.] THE VERTEBRATE EAR. 193 



forwards, growing in the direction indicated by this spiral twist ; 

 i.e. downwards, forwards, and outwards. The cochlear ganglion 

 accompanies it with its nerve cells in intimate contact with the 

 columnar epithelial cells of the thickened floor of the canal. 

 All these parts, nervous and sensory, develop coiisecutively from 

 the base to the apex by interstitial as well as apical growth. 

 There is an interstitial increase in length, due to increase in 

 size of the elements composing the organ, and new elements 

 are added throughout the growing organ. The apical portion is 

 always much younger and more immature than the other parts, 

 as, for example, in an 8.5 cm. Sheep embryo the cochlea has 

 made two and one-half turns. The scalas tympani and vestibuli 

 are beginning to develop. The apical end of Corti's organ is 

 in the epithelial ridge stage, while the basal part of the cochlea 

 has already developed its rods, hair cells, and other structures 

 characteristic of the adult condition. 



In cross-section the cochlear tube is at first flat, with a slit- 

 shaped lumen, but the canal soon enlarges and becomes irregu- 

 larly oval, the shape of the outline of the cross-section varying 

 in different parts of the canal in any given period of growth. 



There is a great difference in thickness between the epithelium 

 of the floor and roof of the canal in the embryo without sharp 

 separation from the side walls, but this character is greater in 

 adults, when also the separation of the thickened floor from the 

 walls becomes more evident. The epithehal ridges, as they 

 grow, become separated by a distinct groove between them. 



At this stage (PI. Ill, Fig. i) the organ has reached very 

 nearly its mature condition as regards the presence of the essen- 

 tial features of its anatomy in the adult, but certain parts still 

 remain which form the most positive evidence of its derivation 

 from a more extensive and less specialized plate of sensory cells 

 or macula acustica. This evidence is seen in the persistence 

 of the primitive hair plate, and the division of the nerve supply 

 between the Sauropsid organ remnant and Corti's organ. It 

 may with appropriateness be called the saurian stage, owing to 

 its great resemblance to the cochlear organ of the adult Alligator. 

 In a transverse section of the cochlear organ of this stage, we 

 have a repetition, part for part, of the Alligator's cochlear organ, 

 and an essential agreement of the histological characters of the 

 latter, although the mammal is born and is exercising its mam- 



