792 APPENDIX. 



produced from the cellular epidermis of the mantle, and subsequently 

 breaks up into the characteristic, large, transversely striated prisms. 



3. The enamel is neither the result of the modification of the primary 

 "dentine," nor superimposed on this from the enamel organ, but a ter- 

 tium quid, the product of the growth and metamorphosis of that exces- 

 sively 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 deiitine, 

 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 membrane, 

 outer layer of the cuticle, whose resemblance to Nasmyth's membrane 

 cannot be overlooked. Now, if we trace the development 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 memhrana preformativa of the hair pulp. 

 Passing from the base towards the apex of the hair, the deep endoplasts 

 become surrounded 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 membrane) 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, page 181), 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 endo- 

 plasts 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 laminoe 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. 

 Now in the long hairs we have a relation of the outer cuticle to the 

 cortex very similar to that of Nasmyth's membrane to the dentine before 

 the development of the enamel, and the conclusion is obvious, that as 

 the development of the inner layer of the cuticle takes place by the dif- 

 ferentiation of the intermediate substance between the cuticle and cortex, 



