The Eetina and Optic Ganglia in Decapods, especially in Astacus. 61 
condition in Branchipus. At an early stage in the growth of the 
optic lobes in Astacus^ as the researches of Reichenbach show, an 
area of celi proliferation appears in a position corresponding to that 
seen permanently in Branchijms. This proliferation is in fact accom- 
panied by a temporary Involution, and the cells resnlting from it 
go in part into the growing retina and in part into the optic ganglia. 
The same is true of the optic lobes in Homarus (cf. Parkee, 90, 
pag. 34), except that in this instance no involution has been observed, 
and the conditions in this respect are almost identical with those in 
Branchipus. Eventually in Homarus and probably also in Astacus 
the ganglia lose their connection with this centre of growth, which 
continues as a growing area for the retina only. It is incidentally 
of interest to observe that, of the two conditions shown by the 
decapods, the one in which there is a simple cell proliferation, as 
exemplified in Homarus , reproduces the primitive condition in Bran- 
chipus more accurately than the involution in Astacus does: in the 
latter a centre of celi proliferation has apparently beeu replaced by 
an involution cenogenetically acquired rather than the reverse. 
The last important point of comparison in the optic tracts of 
Branchipus and Astacus pertains to the direction of growth in the 
different parts. The third and fourth ganglia in Astacus^ as already 
indicated, bave no homologues in Branchipus and it is necessary, 
therefore, to consider only the first and second ganglia. Admittiug 
that . the dorsal side of the stalk in Branchipus corresponds to the 
anterior side in Astacus^ the direction of growth in the retina and 
the first ganglion is the same in both, namely dorsal in Branchipus 
and anterior in Astacus. In the second ganglion, however, a very 
different condition of afPairs obtains, for this ganglion in Astacus^ at 
least so far as its »Punktsubstanz« is concerned, grows posteriorly, 
a direction, mutatis mutandis, precisely contrary to that taken by 
its homologue in Branchipus. Coupled with this peculiarity in the 
growth of the second gauglion in Astacus^ is the fact that the fìbres 
which enter or leave it are involved in either the first or the second de- 
cussation, an arrangement altogether different from that in Branchipus^ 
where, as before mentioned, there is no indication of a true decus- 
sation. If, then, with a change in the direction of growth in a 
ganglion, decussations appear on either side of it, one would natar- 
ally suspect that the new method of growth had some connection 
with the formation of the decussations. Precisely what this connection 
may be can be clearly seen, 1 believe, if one attempts to convert a 
