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 habenule 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 defined 
nerve at all; but the cells of the ganglion habenule 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 Geotria australis. In this species 
the second eye is much better defined than in the European lamprey, 
and its connection with the ganglion habenule 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 habenule and connective tissues are mixed up with the 
nerve-fibres which pass from each eye to its respective optic ganglion. 
Tue Rigut PINEAL Eve or AMMOCCTES, 
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 
