42 
G. H. Parker 
ganglioD, their continuations serving to connect that ganglion witli 
the second one. Grenacher (79, pag. 120) makes a more guarded 
statement when he says that in Mysis the retinal fibres become lost 
in the first ganglion. He is, moreover, nnable to decide whether they 
pass completely through the ganglion or not. To any one that has 
studied ordinary histologieal preparations of these structures, the 
difficulties in the way of deciding such a question must be obvious, 
and it was not nntil I had seen preparations made by the Golgi 
method, as well as those stained in methylen blue, that I was able 
to answer the question to my own satisfaction. In such prepara- 
tions all the stained distal (retinal) fibres ended in a fibrillation in 
the ganglion (PI. 2 Fig. AQ^fbr.r); moreover, all fibres that emerged 
from the proximal side of the ganglion took their origin in fibrilla- 
tions (Fi^. AI ^ fbr.n) ^ and in no case did I observe a fibre that 
passed through the ganglion. These observations lead to the conclu- 
sion that all optic fibres are interrupted in the first optic ganglion. 
It will be remembered that the retinal nerve fibres are really proxi- 
mal processes from the proximal retinular cells and that these cells 
with their processes constitute the most distal set of neurons in the 
optic apparatus of the crayfish. For convenience they may be desig- 
nated as neurons of the first order, while .those fibres that lead 
centrally from the first ganglion mark the beginnings of neurons of 
the second order. The first optic ganglion may, therefore, be defined 
as the structure in which the neurons of the first order end and those 
of the second order begin. 
First Decussation. Although the retinal fibres take the most 
direct course from the retina to the first optic ganglioD, being arranged 
like the radii of a spherical system, the fibres between the first and 
second ganglia have such peculiar relations to the two masses of 
» Panktsubstanz « that, in their passage from one to the other, as 
Berger (78, pag. 199) long ago pointed out, they decussate more or 
less completely. The exact character of this decussatiou, though 
described and figured by several more recent investigators, has never 
been more accurately portrayed than by Grenacher (79, pag. 120), 
whose account I can confirm in every respect. 
In the crayfish the decussation is most satisfactorily studied in 
sections made through the axis of the optic stalk in its anteroposte- 
rior plane (PI. 1 Fig. 27). In such a section the first and second 
ganglia would be seen cut from their anterior to their posterior 
edges. The arrangement of fibres is such that one which Starts from 
