ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 939 



sucli), all the rods are horizontal ; in the full-grown larvee the smaller 

 outermost rods become upright, but those that are larger and deeper 

 retain their horizontal position. In eye V. there is at first a strong 

 tendency to form horizontal rods, but the laterally flattened optic vesicle 

 expands and forms a spacious cavity in the vesicle ; all the rods, except 

 those of the median row of gigantic cells, become upright. In eye VI., 

 which has no median row of gigantic cells, no horizontal rods are 

 formed. 



The outer wall of the optic vesicle in eyes I.-IV. seems to be absent, 

 its presence in the embryo being only indicated by a few characteristic 

 nuclei between the retina and the corneagen. In eye V. the outer wall of 

 the optic vesicle is represented by two great masses of inverted, rod- 

 bearing cells, which are probably derived from two corresponding sensory 

 spots. In eye VI. the outer wall is composed of a thin nucleated 

 membrane, and a cluster of inverted retinal cells derived from a sense 

 organ. 



Eye I. is composed of at least nine sensory spots, four of which, with 

 their central nucleus and median row of giant cells, give rise to the hori- 

 zontal retina ; four more, exactly like the first, give rise to the vertical 

 retina, and the ninth spot to the appendage. All these sense spots unite 

 to form a simple homogeneous organ ; but, during the later stages, the 

 three groups of sensory spots become greatly modified, so that in the 

 adult eye the parts to wliich they give rise are very diiferent in structure. 

 All the retinae are composed of retinophorse, formed by the union of two 

 cells ; they contain two nuclei and two rods, and are supplied with axial 

 and external nerve-fibres. Ganglionic cells are rarely found in the retina 

 of Acilius. The rods are arranged in pairs, which form a mosaic of 

 hexagonal figures when upright, and straight vertical lines when hori- 

 zontal. In both sets the retinidial fibrillas are set at right angles to the 

 rays of light. 



All the larval ocelli of Acilius and of Dytiscus contain more or less 

 distinct dimorphic retinal cells. The giant cells always form a double 

 row along the bottom of the furrow ; their free ends are bent at right 

 angles, and bear short, but broad, horizontal rods. The ends of the 

 smaller retinal cells, and, consequently their rods, may be horizontal, up- 

 right or inverted. Between the two rows of giant rods are two sheets 

 of coarse, vertical nerve-fibres and a layer of medulla-like substance. 

 The pigment granules are deposited on the surface of the retinophorge 

 and around the external nerve-fibres. 



All the eyes are developed from the optic plate — the thickened distal 

 edge of the cephalic lobes. On the proximal edge of this optic plate is 

 a semicircular furrow, which gives rise to the optic ganglion. The furrow 

 is deepened to form two distinct pockets which give rise to the first and 

 second segments of the optic ganglion. The third segment is formed by 

 an inward proliferation on the proximal side of the third segment of the 

 optic plate. The innermost walls of the ganglionic segments are, from 

 the earliest stages, connected with the inner face of the optic plate. 

 Numerous ganglionic cells arise from the optic thickening, and wander 

 along the optic nerves into the optic ganglion. Towards the close of 

 this process, at about the time when the invagination of the sensory 

 areas begins, enormous tripolar cells are formed in each eye, which pass 

 along the optic nerve from the eye to the optic ganglion, dividing rapidly 

 on the way, and producing small tripolar ganglion-cells. Only one of 



