494 EDGAR J. ALLEN. 
even more conclusive in showing that the cylindrical form is the 
normal condition of the fibre. In the lobster embryos, where 
I have been able to identify a good many different elements, it 
has been observed that the same fibre, although generally 
cylindrical, may at times appear beaded, and that a particular 
fibre may be cylindrical when it first stains, but afterwards 
become beaded. This latter fact, which can be readily ob- 
served, places the question beyond doubt. That the result, 
however, can be produced by mechanical means, and is not 
always entirely due to the reagent, is shown by the following 
observation. In the lobster embryo there is a single pair of 
fibres running along the whole length of the cord, the so- 
called giant fibres, which have a diameter very much greater 
than any other fibres of the cord. These can be seen quite 
clearly in fresh embryos, before staining has taken place, and 
are generally cylindrical. In an embryo, however, which has 
been roughly handled in the preparation, I have observed that 
these fibres, whilst still unstained, are distinctly beaded. 
The rapidity with which a fibre beads appears to depend toa 
large extent upon its absolute diameter, and this point is of 
great importance in selecting suitable objects upon which to 
study the histology of the nervous system. The fine branches 
are always the first to assume the varicose condition, and in 
them the process of beading is carried to such an extent that 
the fibre breaks up entirely into drops, which do not even hold 
together. For this reason it is often difficult to obtain stain- 
ing of the whole of an element in the lobster embryo at the 
same time, the finer branches being completely broken up into 
drops before the cell has commenced to stain. The finer longi- 
tudinal fibres also become beaded much more quickly and more 
frequently than the coarser ones. 
In the larve of the smaller Decapods, as for example Pale- 
mon, Palemonetes, &c., where the fibres are all very fine, it 
becomes impossible to obtain useful preparations on account of 
the rapidity with which the beading takes place. The whole 
of the neuropile rapidly becomes converted into a mass of small 
blue drops, which appear to have lost all continuity. Hence 
