It will be observed that the blood corpuscles, the epithelial tissues, the ganglionic corpuscles, the ova and the spermatozoa, are all demonstrably nucleated cells, more or less modified. The first form of connective tissue is so similar to epithelial tissue, that it may obviously be regarded as an aggregate of as many cells as it presents nuclei, the matrix representing the more or less modified and confluent bodies of the cells, or products of these. But if this be so, then the second and third forms have a similar composition, except so far as the matrix of the cells has become fibrillated, or vacuolated, or marked off into masses corresponding with the several nuclei. By a parity of reasoning, muscular tissue may also be considered a cell aggregate, in which the inter-nuclear substance has become converted into striated muscle; while, in the nerve fibres, a like process of metamorphosis may have given rise to the pellucid gelatinous nerve substance. But, if we accept the conclusions thus suggested by the comparison of the various tissues with one another, it follows that every histological element, which has now been mentioned, is either a simple nucleated cell, a modified nucleated cell, or a more or less modified cell aggregate. In other words, every tissue is resolvable into nucleated cells. {191} [Illustration: FIG. 56.—_Astacus fluviatilis._—The structure of the cuticle. _A_, transverse section of a joint of the forceps (× 4); _s_, setæ; _B_, a portion of the same (× 30); _C_, a portion of B more highly magnified. _a_, epiostracum; _b_, ectostracum; _c_, endostracum; _d_, canal of seta; _e_, canals filled with air; _s_, seta. _D_, section of an intersternal membrane of the abdomen, the portion to the right in the natural condition, the remainder pulled apart with needles (× 20); _E_, small portion of the same, highly magnified; _a_, intermediate substance; _b_, laminæ. _F_, a seta, highly magnified; _a_ and _b_, joints.] {192} A notable exception to this generalisation, however, obtains in the case of the _cuticular structures_, in which no cellular components are discoverable. In its simplest form, such as that presented by the lining of the intestine, the cuticle is a delicate, transparent membrane, thrown off from the surface of the subjacent cells, either by a process of exudation, or by the chemical transformation of their superficial layer. No pores are discernible in this membrane, but scattered over its surface there are oval patches of extremely minute, sharp conical processes, which are rarely more than 1‐5,000th of an inch long. Where the cuticle is thicker, as in the stomach and in the exoskeleton, it presents a stratified appearance, as if it were composed of a number of laminæ, of varying thickness, which had been successively thrown off from the subjacent cells.