BIOPLASM IN CUTICLE OF DIFFERENT AGES. 77 



material. They have ceased to grow, and are no 

 longer capable of growth. They are dead. 



117. Bioplasm in Cuticle of different ages. If equal 

 portions of the most superficial and deepest strata of 

 cuticle be taken, the proportion of bioplasm to the 

 formed material will be found much less in the former 

 than in the latter. At the deep aspect, where the 

 elementary parts are being produced, the bioplasm is 

 abundant. Upon the surface, where only old ones, 

 which are about to be cast off, are found, the bioplasm 

 is reduced to a minimum, or has altogether disappeared. 

 Consider how these particles of cuticular epithelium 

 grow. Here is a little mass of bioplasm which grows 

 and then divides into two ; each of these subdivides, 

 and so on. Now, each of these little bodies absorbs 

 nutriment from the surrounding fluid. It increases 

 in size. The older particles on its surface are altered, 

 and appear to be converted into a hard substance, 

 which is improperly described as a membrane (cell- 

 wall). As it approaches the surface, the hard ma- 

 terial, cuticle, deposited layer within layer, becomes 

 thicker and thicker, until at length the mere trace of 

 bioplasm which remains in the centre, being too far 

 removed from the source of nutritive supply to in- 

 crease, perishes ; and the elementary part, in the form 

 of a flattened scale of dry cuticle, having by this time 

 reached the surface of the body, is cast away, while its 

 place is taken by others which grow up from below. 



118. Hair, horn, nail. Hair, horn, and nail are 

 epithelial structures, and if we examine the cells or 

 elementary parts at the bulb, base, or growing portion, 

 we shall invariably find numerous small masses of 

 bioplasm like those situated at the deep aspect of 

 cuticle. It is in this situation that the bioplasm 

 divides and subdivides, and the new cells which are 

 formed push before them those previously developed. 

 In the fully formed hardened cells of these structures 

 the bioplasm has completely disappeared, but in 



