168 FRANCIS VILLY. 



which it is destined to be associated, will be placed. A short time 

 after the canals have been established this patch divides into 

 two, of which one goes to form the sensory epithelium of each 

 ampulla. 



The sensory epithelial areas of the cochlea and sacculus remain 

 continuous with each other for some time. When the sacculus and 

 lagena form, it follows from their position that both are lined by 

 columnar epithelium, the former, however, only in part. The rapid 

 growth of the sacculus necessitates the drawing out of the sensory 

 lining of the floor of the vesicle, so that one end is now situated in 

 the anterior part of the vesicle, and from this point it extends back- 

 wards to the end of the single cochlea pouch present, i.e., the lagena. 

 The first part of the thickened epithelium to be separated from the 

 rest lies on the under surface of the lip of the recessus, and when 

 the pars neglecta of the cochlea developes it necessarily includes it 

 from its formation at the same point. The next sensory tract that 

 differentiates is separated in the position where the remaining pouch 

 of the cochlea (pars basilaris) will ultimately be, and forms the 

 greater part of the wall pushed out in the development of this organ. 

 The lining of the first formed division of the cochlea, i.e., the lagena, 

 is the last to become distinct from the saccular epithelium. This 

 fact may fairly be taken to support my view that the epithelial 

 patches do not develope in the order in which they were evolved. 



I believe that it is of some importance to recognise that the 

 outlying sensory tracts are individualised before the parts to which 

 they are destined to belong. The ampulla and cochlear pouches are 

 provided for in this wajr, all except the lagena ; but this apparent 

 exception does not really contradict the rule enunciated above, as 

 the pouch under consideration is from the first placed in such a 

 position that unless some secondary change were to take place it 

 must be lined by sensory cells as soon as it is formed. The lagena 

 appears early, when the canals have only just been established and 

 when the epithelium of the ampullse is being separated, and it arises 

 in the position where the columnar epithelium is best marked. Thus 

 it resembles the other parts to which the rule applies in possessing 

 sensory cells from the first, whilst it differs from them in not having 

 a sense-organ distinct from that destined for other parts. The 

 ampullae and the remaining two cochlear pouches may be recognised 



