No. I.] EYES OF ARTHROPODS. 1 97 



ganglion-cell. Even at the time when the pupa is ready to 

 come out of its cell, the ganglion-cells of the middle wall, 

 which develops into the enormous optic ganglion of the adult 

 (epiopticon of Hickson), have exactly the same arrangement 

 as that just described, only the main stalks and cells are 

 more numerous and more closely packed (Figs. 5, G,'], op-g/^). 

 The arrangement of the unipolar ganglion-cells upon long 

 stalks, and their method of division, are so evident, both in sec- 

 tions and in macerated specimens, that there can be no doubt 

 concerning the matter. 



Beneath the ganglion-cell layer there is a mass of nerve- 

 fibres which finally develop into the medulla of the optic gan- 

 glion, or the outer medulla (Fig. 7, o.md.). 



The ganglion-cell layer of the middle wall is directly contin- 

 uous with the infolded edge of the outer wall (Figs. 5 and 6) 

 which appears to be a proliferating point, supplying the optic 

 ganglion with new elements (Fig. 5). 



The inner edge of the inner wall {op.g.^) becomes thickened 

 and bent to form a ridge which appears like a papilla in cross- 

 sections. The inner wall, at the base of the papilla, is perforated 

 by a bundle of nerve-fibres arising from the base of the middle 

 wall. This bundle of fibres {n.f. ^) is crossed by a layer of fibres 

 {n.f. ^) arising from the inner face of the ridge, and extending 

 along the under surface of the inner wall, and finally passing to 

 the optic nerve, after mingling with the fibres at the outer edge 

 of the medullary mass. 



In the 10 mm. larvae (Fig. 6), the middle wall, which we 

 shall now speak of as the optic ganglion, has greatly increased 

 in size, and is strongly arched. The outer medulla is composed 

 of two distinct sets of crossing fibres, while the outer wall {op.g}) 

 is reduced to a convex, single layer of cells, from the upper 

 edge of which arises a small tuft of ganglion-cells, which are 

 subsequently added to the optic ganglion. 



The brain-sheath, {br.sh.) has become a stratum of widely 

 separated, columnar cells between two distinct membranes. On 

 the dorsal side of the brain it runs close under the ommateum 

 as a very thin layer, and, bending inwards, terminates at the 

 lower edge of the outer wall (Fig. 6). On the ventral side it is 

 very thick below the outer medulla, but is rapidly reduced to a 

 thin membrane, with an occasional nucleus, beneath the optic 



