62 
G. H. Parker 
second gaüglion of the type in Branchipus into one of the type in 
Astacus. 
In attempting such a conversion it is necessary at the outset to 
appreciate clearly the relations of the various structures involved. The 
second ganglion in Branchipus (PI. 1 Fig. 28) is composed of a dorsal 
layer of ganglionic cells [cl.gn]^ a middle layer of »Piinktsubstanz« 
(II), and a ventral layer of nerve fibres (fbr.n). The first two of 
these grow dorsally at their distal ends. In the second ganglion 
in Astacus^ these three layers reappear. The ganglionic cells (Fig. 27, 
cl.gn) occupy a position corresponding to that of their homologues in 
Branchipus^ and, moreover, grow in a like direction (cf. Fig. 59). The 
»Punktsubstanz« (Fig. 27, I), however, does not lie next the ganglionic 
cells, as in Branchipus^ but projects and grows ventrally, allowing 
the nerve fibres [cx. 1) from the first ganglion to intervene between 
it and the ganglionic cells. These are the more essential dilferences 
between the two types of ganglia, and any change that would lead 
from one type to the other would obviously involve either »Punkt- 
substanz« or nerve fibres. 
Assuming as a beginning a stage in Branchipus in which a few 
of the oldest fibres (the ventral ones) were established between the 
first and second ganglia and in which the proximal end of the 
»Punktsubstanz« of the second ganglion was formed, it is easy to 
point out the paths by which, on the one band, the condition in 
Branchipus will be realised and, on the other, that in Astacus. If 
from this indifferent stage the »Punktsubstanz« grows distally and 
dorsally, it will always remain between the nerve fibres and the 
ganglionic cells as in Branchipus\ if, however, it grows ventrally, 
it will pass between the nerve fibres already formed and finally 
occupy a ventral position as in Astacus. Starting from this indiffer- 
ent stage, then, the condition in either Branchipus or Astacus will 
be arrived at depending entirely upon the direction that this growth 
of the »Punktsubstanz« takes. 
The change in the direction of growth by which the condition 
in Astacus would be brought about would also induce the formation 
of two decussations, one in front of and the other behind the second 
ganglion, precisely as they occur in that crustacean. Thus, in the 
growth of the optic tracts, any newly added fibres in the region 
between the first and second ganglia would necessarily have their 
distal fibrillations in the anterior end of the first ganglion and 
their proximal ones in the posterior end of the second, thus 
