Again and again the aborted character of the eye is stated to be evidence of degeneration in the case of the lamprey. What such a statement means, why the eye is in any way to be considered as aborted, is to me a matter of absolute wonderment: it is true that in the larval form it lies under the skin, but it is equally true that at transformation it comes to the surface, and is most evidently as perfect an eye as could be desired. There is not the slightest sign of any degeneration or abortion, but simply of normal development, which takes a longer time than usual, lasting as it does throughout the life-time of the larval form. Kohl, who has especially studied degenerated vertebrate eyes, discusses with considerable fulness the question of the Ammocoetes eye, and concludes that in aborted eyes a retarded development occurs, and this applies on the whole to Ammocoetes, "but with the important difference that in this case the period of retarded development is not followed by a stoppage, but on the contrary by a period of very highly intensified progressive development during the metamorphosis," with the result that "the adult eye of _Petromyzon Planeri_ does not diverge from the ordinary type." Referring in his summing up to this retarded development, he says: "Such reminiscences of embryonic conditions are after all present here and there in normally developed organs, and by no means entitle us to speak of abnormal development." The evidence, then, is quite clear that the eye of Petromyzon, or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal eye, but simply that its development is slow during the ammocoete stage. The retina of Petromyzon was figured and described by Langerhans in 1873. He describes it as composed of the following layers:-- (1) _Membrana limitans interna._ (2) Thick inner molecular layer. (3) Optic fibre layer. (4) Thick inner nuclear layer. (5) Peculiar double-layered ganglionic layer. (6) External molecular layer. (7) External nuclear layer. (8) _Membrana limitans externa._ (9) Layer of rods. (10) Pigment-epithelium. {95}[Illustration: FIG. 41.--RETINA AND OPTIC NERVE OF PETROMYZON. (AFTER MÜLLER AND LANGERHANS.) On the left side the Müllerian fibres and pigment-epithelium are represented alone. The retina is divided into an epithelial part, _C_ (the layer of visual rod-cells), and a neurodermal or cerebral part which is formed of, _A_, the ganglion of the optic nerve and, _B_, the ganglion of the retina. 1, int. limiting membrane; 2, int. molecular layer with its two layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear layer; 5, double row of tangential fulcrum cells; 6, layer of terminal retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of rods; 10, layer of pigment-epithelium. _D_, axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather too thick.]