The Eetina and Optic Gauglia in Decapods, especially in Astaciis. 39 
together that it is often impossible to distinguisi! their exact limits, 
whereas, in the other parts of this nerve, each axis cylinder usually 
has a perfectly discrete sheath of its own. In the fresh condition 
the axis cylinders seem to be composed of a homogeneous substance 
in which there is no evidence of fibrillation, but in those treated 
with osmio acid a faint but distinct longitudinal striation is visible. 
The third essential constituent of the ganglia, the »Punkt- 
substanz«, is separated into four masses, which form, so to speak, 
the cores of the four optic ganglia (PI. 1 Fig. 27, I — IV). As is well 
known, this material is composed of a vast number of extremely fine 
fibrils, which, in preparations stained with methylen blue or made 
by the Golgi method, can be shown to result from the repeated 
division of the axis cylinders of the innumerable nerves which enter 
it. In no instance that came under my Observation was there any 
evidence of direct union between different Systems of fibrils; appa- 
rently, fibrils derived from different nerve fìbres are at most only in 
contact with one another. In addition to the fibrillar material, which 
is, of course, the essential portion of the »Punktsubstanz«, the latter 
also contains a few connective tissue cells and a rather rieh supply 
of capillaries. 
Before proceeding to a description of the optic ganglia, it is 
necessary to say a word or so about the general relations of the 
three nervous Clements already described. So far as I am aware, 
ali the functional ganglionic cells in the optic ganglion of the cray- 
fish are unipolar. Moreover, the one process that each of these cells 
gives rise to is always directly continuous with a nerve fibre. This 
process, however,. does not represent one end of the fibre, but rather 
meets that structure somewhere on its length as the upright of a 
lettor T meets the transverse part. The nerve fibre at its two ends, 
which correspond of course to the tips of the arms of the T, breaks 
up into fibrillations thus giving rise to a portion of the »Punktsub- 
stanz«. The nervous fibrillar constituent of the »Punktsubstanz« 
seems always to arise from the repeated division of a nerve fibre, 
and there are no nerve fibres that are not connected with ganglionic 
cells; hence, fibrillae, as well as nerve fibres, may be regarded as 
processes from ganglionic cells. Nervous cells with their appended 
processes, as I bave already mentioned, bave been designated by 
Waldeyer as neurons, and, as this term is a convenient one, I 
shall adopt it in the foUowing description. 
