60 
G. H. Parker 
tracts is simpler than in Astacus. As can be seen in dorsoventral 
sections (PI. l Fig. 28), the retina in Branchipiis continues to grow 
along its dorsal margin, a region that, as Claus (86, pag. 310) has 
clearly shown, has been the region of growth even from the earliest 
stages of development. Further, from this same region, material is 
also added to both ganglionic masses, so that all the nervous struc- 
tures in the stalk, the retina and the two optic ganglia may be said 
to originate from this one centre. In Branchipus^ then, the piane of 
growth is dorsoventral, and the growing ends in the three nervous 
structures just mentioned are the dorsal ones. 
In comparing the method of growth in Branchipus and in Asta- 
cus several important differences will be observed. In Branchipus 
the piane of growth for ali the nervous structures of the stalk is 
dorsoventral; in Astaciis only the optic nerve grows in this way, the 
retina and the first, second and third ganglia growing in the antero- 
posterior piane. This difference, which might at first sight seem 
fundamental, is, however, readily explained if it be admitted that 
the stalk in Astaeus^ together with much of its distai contents, has 
been twisted on its axis, so that what is dorsal in Branchipus has 
come to be anterior in Astacus. Such a change as this, which in 
itself is easily conceivable, would offer a sufficient explanation for 
the fact that the retina and the first three ganglia in Astacus bear 
the same relation to the anterior face of the optic stalk in that animai 
as the corresponding structures in Branchipus bear to the dorsal face 
of the stalk in this form. It would also explain the imperfect de- 
cussation of fibres between the third and fourth ganglia, the region 
in which the twisting is actually felt, as well as the fact that the 
optic nerve, the portion of the optic tracts too deep to be effected 
by the twisting, retains the primitive dorsoventral piane of growth. 
Another difference between Branchipus and Astacus concerns the 
centre s of growth. In Branchipus^ ali the nervous organs in the 
optic stalk grow from a common centre. In Astacus^ there is a 
centre of growth for the retina and for each ganglion: thus, the 
retina grows from the hypodermis, and each ganglion from the 
ganglionic cells that surround it. This more differentiated cpndition 
of the centres of growth in Astacus offers, however, no serio us ob- 
stacle to the comparison of the sets of organs in the two animals, 
but is in harmony with the more complex Organisation in Astacus, 
In the development of the optic stalks, Astacus^ together with other 
decapods, passes through a stage that strongly recalls the permanent 
