214 



PATTEN. [Vol. I. 



rods, separated by a band of deep black pigment. The rods 

 on either side of the median furrow are short, and gradually 

 decrease in length towards the periphery of the retina. 



The peripheral cells of the retina are rodless, and appear to 

 be continuous with a layer of small flattened nuclei outside of 

 those belonging to the corneal hypodermis, and extending out- 

 wards as far as the edge of the lens. The pigment must be 

 removed in order to see this layer of nuclei. The outer part 

 of the eye is surrounded by a delicate membrane. 



The ocelli are formed in the embryos of Hydrophihis by in- 

 vaginations of the ectoderm, directed diagonally inwards. In 

 the just-hatched larvae these pits are apparently still open, 

 although the rods are well developed. The ocellus is com- 

 posed of three distinct layers of cells (Fig. lo), of which the 

 thick inner layer, the retina, is directly continuous on the dor- 

 sal side {ah) with the hypodermis. In the middle of the retina 

 is a deep furrow, in which lies a single (?) row of rods. On the 

 ventral side the edge of the retina is rounded, and, as can be 

 easily seen, is not directly continuous, as on the opposite side, 

 with the hypodermis. It appears to be continuous, however, 

 with a thin layer of cells {vb) between it and the overlying 

 corneagen. In the centre of the eyes this layer {yb) is thick- 

 ened, and contains a cluster of deeply-stained nuclei. The 

 limits of the layer are not very clear ; but, from observations 

 made on other material, I can hardly doubt that it is continu- 

 ous, as I have indicated in the semi-diagrammatic drawing, at z, 

 with the thick and very distinct corneagen which is continuous, 

 on its ventral side, with the unmodified hypodermis. 



It is thus evident, if my observations are correct, that the 

 eye, although practically a closed vesicle, is not so in reality, 

 as is shown by the absence of nuclei at the point z, and the 

 continuity of the three layers. 



In the older larvae of Hydrophihis and Dytisctis, from 5 to 

 8 mm. long (Fig. 8), the retina contains much dark pigment. 

 In the centre of the eye is a core of nuclei {vb) which 

 probably belong to the outer wall of the optic vesicle. Between 

 them and the lens is a distinct layer of cells. 



I have taken special care to see that my sections, both of the 

 larvae and embryos, passed through the axis of the eye, and 

 not to be misled by the deceptive appearances of sections which 



