226 PATTEN. " [Vol. I. 



Fig. 6. Section of a lo mm. larva. The middle wall of the ganglionic fold 

 {op.g^.) now resembles the optic ganglion of the adult. The floor of the optic 

 invagination has developed into the ommateum, divided into a dorsal and ven- 

 tral half by a raphe of irregularly arranged nuclei («). The ommateum is 

 bounded bj a dorsal and ventral fold of the ectoderm {d.f. and v./.), which, 

 in the succeeding stage, unite with each other to form a double layer of cells 

 over the ommateum. The retinal ganglion (ri-^.) appears in this stage as a 

 cone of straight nerve-fibres interspersed with nuclei. 



Fig. 7. Section through the head of a young pupa. The optic ganglion 

 (op.g^.) has nearly completed its development. The outer wall of the gan- 

 glionic fold {pp.g^.^ has almost disappeared, while the inner wall is breaking 

 up into unipolar ganglionic cells continuous with those forming the cortical 

 layer of the brain. The mass of nerve-fibres {n.f.^, of Fig. 6) has developed 

 into the inner medulla, i.nid. 



Fig. 8. Cross-section through the head of Acilius, showing the posterior 

 dorsal ocellus with its dorsal extension. It is important to notice the exten- 

 sion of the nuclei of the corneal hypodermis over the peripheral parts of the 

 retina. 



Fig. 9. Section through the eye of a young Hydropkilus larva, showing 

 the corneal hypodermis {c.hy.^^ and the cluster of nuclei {y.b.') which prob- 

 ably belong to the outer wall of the optic vesicle. 



Fig. 10. Section, semi-diagrammatic, through the ocellus of a just-hatched 

 Hydropkilus larva. The ocellus contains but a single row of rods, and is 

 still open at z, although practically closed. The hypodermis at ab.^ on the 

 dorsal side of the ocellus, thickens, while it is still continuous with the retina, 

 and gives rise to the "dorsal extension" which becomes double-layered by 

 the subsequent growth of the edge of the corneagen over it. 



