No. I.] 



EYES OF ARTHROPODS. 



225 



EXPLANATION OF PLATE. 



a. Raphe of indifferent cells in the 



ommateum. 

 ab. Dorsal extension of retina. 



b. Ganglionic(?) cells in the retina. 

 br. Brain. 



br.sk. Brain-sheath. 



c.f. Layer of cross fibres beneath the 



retinal ganglion. 

 c.Jty. Corneagen. 

 d.f. Dorsal fold of comp. eye. 

 E. Optic thickening. 

 ft.c. Fat cells. 

 g.c.^ Large unipolar ganglionic cells 



at inner edge of optic ganglion. 

 g-.c.br. Ganglionic cells of the brain. 

 ky. Hypodermis. 

 t.ck. Inner chiasma. 

 t.7nd. Inner medulla. 

 inf.g.l. Inferior ganglionic cells. 

 n. Nerve of the dorsal extension. 

 n.b.l. Nerve-bundle layer. 

 n.f.^ and n.f."^ Decussating nerve- 

 fibres. 

 o.ck. Outer chiasma. 

 o.md. Outer medulla. 



op.g.^ Outer wall of ganglionic fold. 

 op.g."^ Middle wall of ganglionic fold. 

 op.g.^ Inner wall of ganglionic fold. 

 op.n. Optic nerve of Vespa. 

 op.n.^ Optic nerve of Acilius. 

 rd. Rods. 



rd.^ Large median rods. 

 rd.^"^ Rods of the dorsal extension. 

 rt.f. Retinophorae. 

 ri.g. Retinal ganglion. 

 rt.g."^ Cortical layer of retinal gan- 

 glion. 

 rt.g.^ Nerve-spindle layer of retinal 



ganglion. 

 v.b. Outer wall of optic vesicle. 

 v.f. Ventral fold of compound eye. 

 X. Connection of optic nerve with 



the middle wall of ganglionic 



fold. 

 xy. Nuclei imbedded in pigment. 

 y. Edge of the sheet of ectoderm that 



is growing over the brain. 

 z. Dorsal edge of corneagen, when it 



is continuous with the middle 



layer, d.f. 



Figs, i to 6 have been selected from a number of drawings in my possession, 

 representing the development of the compound eye and optic ganglion of 

 Vespa. They have been chosen and reproduced as simply as possible, in 

 order to illustrate the salient features in the development of the parts under 

 consideration. 



Figs. 1 to 6 represent cross-sections of the heads of embryonic and larval 

 Wasps. 



Fig. I. Section of the cephalic lobes of an embryo, whose appendages 

 have just appeared. At y is the sheet of ectoderm advancing towards the 

 optic thickening E, with the edge of which it finally unites. 



Fig. 2. Section of a later stage in which the ectodermic layer (^y) has cov- 

 ered in all the brain except that part which develops into the optic gan- 

 glion (op.g.^^^). The neurilemma {^br.sk.) is forming from a layer of cells 

 split off from the brain. 



Fig. 3. Section of an embryo just ready to hatch. The sheet of ecto- 

 derm {y, of Figs. I and 2) has united with the edge of the optic thicken- 

 ing £. 



Fig. 4. Section from a 3 mm. larva, showing the first stage of the invagi- 

 nation in the optic thickening {E). The brain-sheath (br.sk.) completely 

 covers the optic ganglion and extends along the optic nerve to the eye. 



Fig. 5. Section of a 5 mm, larva. There is an ectodermic fold only over 

 the ventral edge of the invagination. The middle wall of the ganglionic 

 fold ipp.g.^) has greatly increased in size. 



