{80}In a series of sections I have followed the nerve of the right pineal eye to its destination, as described in my paper in the _Quarterly Journal of Microscopical Science_, and have found that it enters into the _ganglion habenulæ_ just as the nerve to any simple eye enters into its optic ganglion. This nerve, as I have shown, is a very distinct, well-defined nerve, with no admixture of ganglion-cells or of connective tissue, very different indeed to the connection between the left pineal eye and its optic ganglion. Here there is no defined nerve at all; but the cells of the _ganglion habenulæ_ stretch right up to the remains of the eye itself. Seeing, then, that both the eye and ganglion on this side have reached a much further grade of degeneration than on the right side, it may be fairly concluded that the original condition of these two median eyes is more nearly represented by the right eye, with its well-defined nerve and optic ganglion, than by the left eye, or by the eyes in lizards and other animals which do not show so well-defined a nerve as is possessed by Ammocoetes. Quite recently Dendy has examined the two median eyes in the New Zealand lamprey _Geotria australis_. In this species the second eye is much better defined than in the European lamprey, and its connection with the _ganglion habenulæ_ is more nerve-like. In neither eye, however, is the nerve so clean cut and isolated as is the nerve of the dorsal, or right, eye in the Ammocoetes stage of _Petromyzon Planeri_; in both, cells resembling those of the cortex of the _ganglion habenulæ_ and connective tissues are mixed up with the nerve-fibres which pass from each eye to its respective optic ganglion. THE RIGHT PINEAL EYE OF AMMOCOETES. The optic fibres of the right median eye of Ammocoetes are connected with a well-defined retina, the limits of which are defined by the white pigment so characteristic of this eye. This pigment is apparently calcium phosphate, which still remains as the 'brain-sand' of the human pineal gland. The cells, which are hidden by this pigment, were described by me in 1890 as the retinal end-cells with large nuclei. In 1893, Studniçka examined them more closely, and concluded that the retinal cells are of two kinds: the one, nerve end-cells, the sensory cells proper; the other, pigmented epithelial cells, which surround the sense-cells. The sense-cells may contain some of the white pigment, but not so much as the other cells. Similarly, in the {81}median eyes of Limulus, Lankester and Bourne find it difficult to determine how far the retinal end-cells contain pigment and how far that pigment really is in the cells surrounding these nerve end-cells.