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 nascentie and 
soft, become firmly connected with the latter; and if at the 
same time the separate coils of the spiral fibre le 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 bemg 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 
