Next comes the ganglion of the retina (B, Fig. 41) (Parker's first optic ganglion), the cells of which form the small bipolar granule-cells of the inner nuclear layer; granule-cells arranged in rows just as they are shown in Claus' picture of the same layer in the retina of Branchipus (Fig. 40), just as they are found in the cortical layers of the optic ganglion of the pineal eye (_ganglion habenulæ_), in the { 98}optic lobes and other parts of the Ammocoetes brain, or in the cortical layers of the optic ganglia of all arthropods.
Between this small-celled nuclear layer (4, Fig. 41) and the layer of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear layer), we find in the eye of Ammocoetes and Petromyzon two well-marked rows of cells of a most striking character--viz. the two remarkably regular rows of large epithelial-like cells with large conspicuous nuclei, which give the appearance of two opposing rows of limiting epithelium (5, Fig. 41), already mentioned in connection with the researches of Langerhans and W. Müller. Here, then, is a striking peculiarity of the retina of the lamprey, and according to Müller the obliteration of these two layers can be traced as we pass upwards in the vertebrate kingdom. Among fishes, they are especially well seen in the perch; in the higher vertebrates the whole layer is only a rudiment represented, he thinks, by the simple layer of round cells which lies close against the inner surface of the layer of terminal fibres (Nervenansätze), and is especially evident in birds and reptiles. In man and the higher mammals they are probably represented by the horizontal cells of the outer part of the inner nuclear layer.
Seeing, then, that they are most evident in Ammocoetes, and become less and less marked in the higher vertebrates, it is clear that their origin cannot be sought among the animals higher in the scale than Ammocoetes, but must, therefore, be searched for in the opposite direction.
Müller describes them as forming a very conspicuous landmark in the embryology of the retina, dividing it distinctly into two parts, an outer thinner, and an inner somewhat thicker part, the zone formed by them standing out conspicuously on account of the size and regularity of the cells and their lighter appearance when stained. Thus in his description of the retina of an Ammocoetes 95 mm. in length, he says, "The layer of pale tangentially elongated cells formed a double layer and produced the appearance of a pale, very characteristic zone between the outer and inner parts of the retina."