IE? 6 THE EYES OF ARTHROPODS. 



impregnated with methylene blue, only the terminals of the optic nerves are stained. 

 They are then seen as small bundles passing in definite order through the gang- 

 lion, between the medullary core and the nerve cell layer. (Fig. 51.) Each 

 bundle of fibers, op.f., probably represents the terminals of a definite ommatidium. 

 On reaching the proximal edge of the lobe, two delicate fibers are given off, one 

 on each side, that penetrate the first core and end there in a few straggling 

 branches. (Fig. 51, A, op.f.) Just beyond them, a compact tuft of varicose 

 fibers is formed on the proximal outer surface of the core. The main fiber then 

 passes to the under surface of the second core, forming with other fibers a 

 characteristic chiasma, x., and then, bending upward, ends in widely distributed 

 dendrites. 



op.n 



FIGS. 107-108. Diagrams to illustrate the relation between the brain, optic ganglia and lateral eyes of an 

 arthropod and a vertebrate. In both cases the parts are projected onto the same transverse plane. FIG. 107. 

 arthropod. FIG. 108. vertebrate, where the same parts, by the infolding of the medullary plate, have been 

 transferred to the walls of the cerebral vesicles. The optic ganglia are inverted, forming the roof of the mid- 

 brain; the compound eye, with its visual cells and underlying ganglionic cells, r.I., forms the inverted retina. 



Many other fibers, like the one just described, enter on the opposite side of 

 the first core and extend along its inner face, giving off the varicose dendrites; they 

 then pass to the outer face of the second, ending in the double set of dendrites 

 on its proximal margin. (Fig. 52.) In figure 51, these fibers are seen as dotted 

 strands running diagonally across the inner face of the first lobe, and appearing 

 as continuous strands on the outer surface of the second. On passing from one 

 lobe to the other, the two sets of fibers form the well known chiasma (Fig. 52,^.) 

 When seen from the surface, the whole effect is that of a single lobe that has been 

 twisted, through about one revolution, into two lobes. 



The surface neurones of each lobe send their fibers into the other medullary 

 core. For example, the fiber from cell a (Fig. 52), extends along the outer surface 

 of the second core, parallel with the optic nerve fibers, to the under surface of the 

 first and then upward, ending in the central part of the distal margin of the core. 

 Neurones b take the reverse course. The remaining ones take intermediate 

 courses. 



