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242 CONTRIBUTIONS TO Meyen himself correctly observes, when treating of those spiral tubes whose very narrow fibres lie close upon one another, that an enveloping membrane could not indeed be observed, but that this by no means justified our concluding on its absence. For if the thickenings of the cell-walls which are formed in most, perhaps in all, cases in spiral lines, in those instances in which they make their appearance early, whilst the original cell-wall itself is yet in statu nascentiæ and soft, become firmly connected with the latter; and if at the same time the separate coils of the spiral fibre he perfectly close one upon another, so that with our present microscopes no space remains perceptible between them,—it naturally follows that on tearing the entire membrane (the so-called unrolling of the spiral vessels), the fracture in the direction of the coils of the fibre must be so sharp that our instruments could not possibly show the inequalities. At the same time it should be remembered that the original cell-membrane, especially in long hair-cells, frequently undergoes so great an expansion that it must at last become infinitely delicate, so that even the thinnest and apparently most simple cell-wall does not exclude the possibility of its being composed of the original membrane and the secondary deposit. If, then, we proceed from those spiral cells and vessels whose coils are so far distant from one another as to admit of no doubt with respect to the existence of an external enveloping membrane, and if we trace the presence of this membrane through all the forms of the constantly approximating coils of the fibre, until only the feebleness of our optical instrument renders further direct observation impossible, the laws of sound analogy require that we should, in such instances, also admit the presence of a similar membrane. There is yet a more direct mode of proof, namely, the investigation of the history of the development.

It is an altogether absolute law, that every cell (setting aside the cambium for the present) must make its first appearance in the form of a very minute vesicle, and gradually expand to the size which it presents in the fully-developed condition; an extended investigation of this formative process also invariably shows that a cell never exhibits a trace of spiral formation, discoverable either from its aspect, or on tearing it, previous to its complete development, i.e. before