38 
G. H. Parker 
by the clearness of their outlines in well-stained preparations, are 
distingui shable from the unipolar cells only in that they possess no 
processes. They oceur in regions where the ganglia are still growing; 
for, as will be shown presently, these structures increase by the 
addition of new elements even after the animai has reached maturity. 
It seems to me probable that the apolar cells represent the undifferen- 
tiated material from which unipolar cells will be ultimately formed. 
If this be so, then the apolar cells as such have no physiological 
role to play in the nervous functions of the ganglion. 
The unipolar cells (PI. 2 Figs. 48, 49, 51), which are vastly 
more numerous than the apolar ones, are of an extremely simple 
type. They consist of roundish or oval masses of protoplasm con- 
taining a single relatively large nucleus. Their single process is so 
simple and their surface so smooth that there is nothing that can 
be interpreted in the sense of protoplasmic processes. In this respect 
they agree very closely with the great majority of cells from the 
central nervous system of the crayfish, which, as Eetzius (90, pag. 49) 
has already showD, are, as a rule, of this simple unipolar type. 
Two kinds of unipolar cells, large and small, can be distin- 
guished. Almost all the cells of the first, second, and third ganglia are 
small (PI. 1 Figs. 34, 35, 36, cl.gn) , there' being only a few large 
ones in the last two ganglia named (Fig. 36, x). Conversely, almost 
all the cells of the fourth ganglion are large (Fig. 37, cl.gn). So far 
as I have been able to observe, small cells are always associated 
with fine short fibres, and large cells with coarse and usually long 
fibres. This relation is so invariable that the size of a ganglionic 
cell is a sure indication of the character of the fibre connected with 
it and vice versa ; thus the large ganglionic cells that cover the fourth 
ganglion belong to the fibres that constitute the optic nerve and that 
are the longest fibres in the optic tracts, whereas the small cells that sur- 
round the first, second, and third ganglia are connected with the smaller, 
shorter fibres that intervene between the retina and the fourth ganglion. 
The nerve fibres that enter into the composition of the optic 
ganglia are essentially similar to those found in other parts of the 
crayfish. Each fibre consists of a relatively large axis cylinder sur- 
rounded by a Schwann's sheath, the nuclei of which occur at rather 
irregulär intervals. Different fibres vary considerably as far as the 
development of their Schwann's sheaths are concerned. In one part 
of the optic nerve (PI. 2 Fig. 38, n.d]^ for instance, the] axis cylin- 
ders are nearly devoid of sheaths and are so intimately pressed 
