432 APPENDIX. 



the primary " dentine/' nor superimposed on this from the 

 enamel organ, but a tertium quid, the product of the growth 

 and metamorphosis of that excessively thin layer of organic 

 matter which lies between the dentine and the enamel. 



In support of this view, also, a very close analogy may be 

 found in the mode of development of the shaft of the hair — a 

 structure which exhibits the closest correspondence with the 

 teeth. The fibrous cortex of the hair is in fact homologous 

 with dentine; it is a horny dentine containing rudimentary 

 canals. External to this substance we find two layers, the 

 inner composed of parallel horny structureless plates, closely 

 united and set obliquely on the shaft, in fact, a rudimentary 

 horny enamel ; the outer consisting of a tough areolated mem- 

 brane, outer layer of the cuticle, whose resemblance to Nasmyth/s 

 membrane cannot be overlooked. Now, if we trace the develop- 

 ment of these layers in the long hairs of the head, we find that 

 they pass on the bulb into a structureless limitary membrane, 

 beneath which lie the endoplasts of the pulp ; this is, in fact, a 

 membrana preformativa of the hair pulp. Passing from the base 

 towards the apex of the hair, the deep endoplasts become sur- 

 rounded by the horny matter and the pigment-granules of the 

 cortex, while the superficial layer remains free from the latter, 

 but gradually becomes horny, and loses its endoplasts. Its outer 

 portion then becomes the areolated outer (Nasmyth's mem- 

 brane) cuticular layer, while its inner portion breaks up into 

 the parallel plates of the inner (enamel) cuticular layer. So 

 far as we have been able to observe in the long hair, however, 

 the disappearance of the endoplasts takes place before the 

 areolation and lamination of the periplast which corresponded 

 to them, so that, as we have already stated (note p. 182, vol. I), 

 the cuticle does here pass into an apparently structureless layer. 

 This, however, is not, as it seemed, a real discrepancy from 

 Prof. Kolliker's views, for in the short thick hairs, such as 

 those of the nostril, the endoplasts persist longer, and we see 

 that, as he states, the areolations of the outer cuticle are the 

 representatives of the cell-cavities of the outer layer of the 

 pulp ; while the laminae of the inner layer are the result of the 

 lamination of the next layer of the pulp, whose endoplasts may 

 be seen gradually disappearing, whilst its periplast breaks up 

 into plates. 



