No. 1.] EVES OF ARTHROPODS. 137 
derm, z. It contains in the centre a round medulla, the down- 
ward continuation of the fibrous mass, m. d. in Fig. 35. The 
second section (Fig. 36) passes through about the middle of the 
first and second ocular segment, and just above the third. The 
ring of ganglion cells is now interrupted on its median edge, 
forming a narrow inlet into a central space lined with the 
cut ends of medullary fibrillae. The whole cavity is soon 
filled with a mass of fibres which develop into the medullary 
stalk of the optic ganglion, s. 0. ¢., which afterwards serves to 
connect the different ganglionic centres with the brain. 
In all sections of the optic ganglion during the younger 
stages, its inner face is sharply defined, as though bounded by a 
delicate membrane, something like the basement membrane on 
the inner surface of ectodermic layers. In the older stages this 
membrane disappears, and the optic ganglion becomes a mass of 
loosely connected cells. 
In the section shown in Fig. 36, and in those of the same 
series below it, the short bundles of nerve fibres connect- 
ing the optic ganglion with the eyes, pass completely through 
the ganglionic layer, at the inner surface of which they bend 
upwards, and pass through the centre of the ganglion into the 
brain. 
MEDULL# OF THE OpTic GANGLION. —It is necessary to 
understand the position and arrangement of the larval medullz 
in order to comprehend the structure of the optic ganglion of the 
imago. They become definitely established soon after or during 
the rupture of the embryonic membranes, and may be studied 
in a series of sections (Figs. 42-47) cut from a head like that in 
Fig. 8. 
The first section (Fig. 42) passes through the upper ends of 
eyes I. and III. and cuts through the optic ganglion at its 
junction with the brain, showing the cut ends of the medullary 
fibres as they bend from the brain down into the centre of the 
optic ganglion. After four more sections, we come to one (Fig. 
43) passing through the upper end of the optic ganglion, in 
the centre of which is a circular mass of medullary substance 
bounded by a layer of minute dark cells, continuous with those 
surrounding the medulla of the brain. 
In the fourth next section (Fig. 44) the optic ganglion appears 
as an oval mass of cells close beneath the optic plate. The 
