It is evident from Fig. 41 that the retina of Ammocoetes and Petromyzon differs in a striking manner from the typical vertebrate retina. The epithelial part (C) remains the same--viz. the visual rods, the external limiting membrane, and the external nuclear layer; but the cerebral part, the retinal ganglion (A and B), is remarkably different. It is true, it consists in the main of the small-celled mass known as the inner nuclear layer, and of the reticulated tissue or 'neuropil' known as the inner molecular layer, just as in all other compound retinal eyes; but neither the ganglion cell-layer nor the optic fibre-layer is clearly defined as separate from this molecular layer; on the contrary, it is matter of dispute as to what cells represent the ganglionic layer of higher vertebrates, and the optic fibres do not form a distinct innermost layer, but pass into the inner molecular layer at its junction with the inner nuclear layer. A comparison of this innermost part of the retina (A, Fig. 41), with the corresponding part in Berger's picture of Musca (_n.l.o.g._, Fig. 38), shows a most striking similarity between the two. In both cases the fibres of the optic nerve (_O.n._, Fig. 38) which cross at their entrance pass into the 'neuropil' of this part of the retinal ganglion, and are connected probably (though that is not proved in either case) with the cells of the ganglionic layer. In both cases we find two well-marked parallel rows of cells in this part of the retina, of which one, the innermost, is composed in Ammocoetes of large ganglion-cells, and the other mainly of smaller, deeper staining cells apparently supporting in function. Similarly, also, in Branchipus, as I conclude from my own observations as well as from those of Berger and Claus, the ganglionic layer is composed partly of true ganglion-cells and partly of supporting cells arranged in a distinct layer. This part, then, of the retina of Ammocoetes is remarkably like that of a typical arthropod retina, and forms that part of the retinal ganglion which may be called the ganglion of the optic nerve. 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.