The Retina and Optic Ganglia in Decapods, especially in Astacus. 63 
requiring that the new fibres should cross the course of those already 
established. Under such circumstances, the continuous additìon of 
new fibres wonld only add to the fulness of the decussation. In the 
same way the nerve fibres connecting the second and third ganglia 
would suffer a decussation, since the third ganglion grows in the 
same direction as the first. Thus both the first and second decus- 
sation s may be said to be the result of the change in the direction 
of growth in the second ganglion. 
If this explanation of the origin of the decussations is true, it 
follows that the two decussations should lie in the same piane and 
that this piane should coincide with the piane of growth in the 
given part of the optic stalk. As the preceding account has shown, 
such is really the case: in both decussations in Astacus the fibres 
cross in the anteroposterior piane and this piane is also the piane 
of growth for the optic structures of that region. Moreover, this 
explanation makes clear the course of certain anomalous nerve fibres 
that have been observed in the second ganglion. These fibres (PI. 2 
Fig. 57 x) connect what is ontogenetically the older part of the first 
ganglion with the corresponding part of the second one, and repre- 
sent the fibres between which the «Punktsub stanzte might be expected 
to grow in gaining the position characteristic of it in Astacus. Such 
fibres, as figure 57 shows, are either involved in the »Punktsubstanz« 
or lie posterior (= ventral in Branchipus) to it, thus recalling the 
primitive position occupied by these fibres in Branchipus. These 
observations lead me to believe that the explanation which I have 
offered for the two decussations is a true one. 
What could have led to the change in the direction of 
growth in the second ganglion is a question that I cannot defi- 
nitely answer. Possibly the crowding of the first and second ganglia 
on one another, as new nervous Clements were added, pressed the 
)) Punktsubstanz « of the second ganglion into a new channel. But 
such answers can be at most only suggestive. That the change in 
the direction of growth in the second ganglion is an adequate ex- 
planation of the two adjoining decussations seems evident enough. 
Whether the cause of this change can be discovered or not is another 
question. 
