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 Ammocœtes, is in no sense an abnormal eye, but simply that its development is slow during the ammocœte 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.