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 nascentia and 

 soft, become firmly connected with the latter; and if at the 

 same time the separate coils of the spiral fibre lie perfectly 

 close one upon another, so that with our present microscopes 

 no space remains perceptible between them, — it naturally fol- 

 lows that on tearing the entire membrane (the so-called un- 

 rolling 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 appear- 

 ance 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 



