Fig. 34.—Horizontal Section through Brain of Ammocœtes, to show the Left, or Ventral Pineal Eye.
pn.2, left or ventral pineal eye; pn.1, last remnant of right, or dorsal pineal eye; g.h.r., right ganglion habenulæ; g.h.l.1, g.h.l.3, parts of left ganglion habenulæ; pi., fold of pia mater which separates the left ganglion habenulæ from the left pineal eye; f., strands of nerve-fibres connecting the left eye with its ganglion, g.h.l.3; V3, third ventricle; V.aq., ventricle of aquæduct.
The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in appearance, being much less evidently an eye. Fig. [34] is one of the same series of horizontal sections as Fig. [31], pn.1 being the last remnant of the right, or dorsal, eye, while pn.2 shows the left, or ventral, eye with its connection with the left ganglion habenulæ.
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 Ammocœtes. 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 Ammocœtes 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 Ammocœtes.
The optic fibres of the right median eye of Ammocœtes 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 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.
The interior of the eye presents the appearance of a cavity in shape like a cornucopia, the stalk of which terminates at the place where the nerve enters. This cavity is not empty, but the posterior part of it is filled with the termination of the nerve end-cells of the retina, as pointed out by me and confirmed by Studniçka. These terminations are free from pigment, and contain strikingly translucent bodies, which I have described in my paper in the Quarterly Journal, and called rhabdites, for they present the same appearance and are situated in the same position as are many of the rhabdites on the terminations of the retinal end-cells of arthropod eyes. Studniçka has also seen these appearances, and figures them in his second paper on the nerve end-cells of the pineal eye of Ammocœtes.
Up to this point the following conclusions may be drawn:—
1. Ammocœtes possesses a pair of median eyes, just as was the case with the most ancient fishes, and with the members of the contemporary palæostracan group.
2. The retina of one of these eyes is well-defined and upright, not inverted, and therefore in this respect agrees with that of all median eyes.