The Retina and Optic Ganglia in Decapods, especially in Astacus. 43 
the extreme posterior edge of the first ganglion passes to the extreme 
anterior edge of the second gauglion; and, conversely, one from the 
extreme anterior edge of the first ganglion passes to the extreme 
posterior edge of the second one ; the two fibres thus cross one 
another like the arms of a lettor X. The other fibres occupy inter- 
mediate positions, i. e., in proportion as the fibre's point of origin in 
the first ganglion is removed from the anterior end of that ganglion , 
so is its point of Insertion in the second ganglion removed from the 
opposite end of the second ganglion. It follo ws from this that a fibre 
Coming from the centre of the first ganglion should enter the centre 
of the second one, and, as a matter of fact, such is the case. These 
relations will perhaps become clearer on Consulting the diagrammatic 
figure 59 (PI. 3), as well as figure 57 (PI. 2), in which the courses of 
a few fibres are giyen. 
The decussating fibres form bundles that lie almost entirely in 
the anteroposterior piane, and consequently the decussation is best 
seen in sections cut in this piane (PI. 1 Fig. 27) ; when the ganglia 
are cut dorsoventrally (Fig. 26) , all traces of the decussation are 
lost, as is also the case in transverse sections of the fibrous regions 
(PI. 1 Fig. 34, ex. 1). Such sections when more highly magnified 
(PI. 2 Fig. 45) show usually some bundles of fibres parallel to the piane 
of section and others cut either transversely or more or less obliquely. 
Ali the fibres that enter into the distai decussation bave their 
origins, as I mentioned before, in the »Punktsubstanz« of the first 
ganglion (PI. 2, Fig. 47, fbr.n) , and, so far as my observations go, 
they ali terminate in fibrillations in the second ganglion. Each fibre 
is attached to a ganglionic celi (PI. 2 Figs. 48, 49) , and these cells 
together constitute the horseshoe - shaped mass that surrounds the 
second ganglion (PI. 1 Fig. 35). The cells and fibres bave a rather 
definite arrangement, which has been represented in the diagram- 
matic figure 59. The fibres that arise from the posterior end of 
the first ganglion bave their cells in the most proximal portion of 
the cellular mass, and those from the anterior portion of the gang- 
lion are connected with cells in the distai portion of the mass. 
Fibres intermediate in position are, of course, united with cells in 
a middle place in the cellular mass. Although the great majority 
of cells are of the small type (PI. 2 Fig. 48), the proximal portion of 
the ganglion contains usually a number of the larger kind of cells 
(PI. 2 Fig. 49). These are connected with large nervo fibres that extent 
from the posterior end of the first ganglion to the anterior endofthe second. 
