. Text-book of embryology. Embryology. AM. Fig. -Illustrating the development of the rods in Lepidosiren. The upper side of the figures represent the side turned away from the leDS. A, B, C, D from stage 35 ; E, fully developed visual cell at stage 38, fixed during exposure to light; E*, similar element killed in the dark. , annular vacuole; , fatty globules stained black by osmic acid ; , external limiting membrane ; n, nucleus of visual cell; r, rod. waves into nerve impulses—are at the ends of the cells which point away from the lens. To reach these rods and cones the light ra
. Text-book of embryology. Embryology. AM. Fig. -Illustrating the development of the rods in Lepidosiren. The upper side of the figures represent the side turned away from the leDS. A, B, C, D from stage 35 ; E, fully developed visual cell at stage 38, fixed during exposure to light; E*, similar element killed in the dark. , annular vacuole; , fatty globules stained black by osmic acid ; , external limiting membrane ; n, nucleus of visual cell; r, rod. waves into nerve impulses—are at the ends of the cells which point away from the lens. To reach these rods and cones the light rays have therefore to traverse the whole thickness of the retina. This remarkable arrangement of the retina, precisely the opposite of what we should expect, is one of the characteristic features of Vertebrates. Its morphological significance is however at once made clear by a consideration of the main facts of development of the eye as already outlined. These, in fact, show that what becomes the proximal surface of the retina, the surface which faces away from the lens, was originally part of the inner surface of the brain rudiment and therefore of the outer surface of the ectoderm before it became involuted to form the brain. The visual cells develop therefore on what was originally a part of the outer surface of the body and their rods point in a direction which was originally outwards. The mode in which the rods develop is illustrated by Fig. 76 which is taken from Lepidosiren (Graham. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869-; Heape, Walter, 1855-1929. London : Macmillan
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