80 THE ORIGIN OF VERTEBRATES 



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 habenulce just as the nerve to any simple eye enters 

 into its optic ganglion. This nerve, as I have shown, is a very dis- 

 tinct, 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 denned 

 nerve at all ; but the cells of the ganglion habenulce 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 degenera- 

 tion than on the right side, it may be fairly concluded that the 

 original condition of these two median eyes is more nearly repre- 

 sented by the right eye, with its well-defined nerve and optic gang- 

 lion, 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 

 Ammoccetes. Quite recently Dendy has examined the two median 

 eyes in the New Zealand lamprey Gcotria australis. In this species 

 the second eye is much better defined than in the European lamprey, 

 and its connection with the ganglion habenulce 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 Ammoccetes stage of Petromy- 

 zon Planeri; in both, cells resembling those of the cortex of the 

 ganglion habenulce and connective tissues are mixed up with the 

 nerve-fibres which pass from each eye to its respective optic ganglion. 



The Eight Pineal Eye of Ammoccetes. 



The optic fibres of the right median eye of Ammoccetes are con- 

 nected 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 pig- 

 ment, were described by me in 1890 as the retinal end-cells with large 

 nuclei. In 1893, Studnicka examined them more closely, and con- 

 cluded 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 



