. The advancement of science. Occasional essays & addresses. Natural history. 38 DEGENERATION : shape, wliilst tlie cloacal chamber forms. The brain remains quite small and undeveloped, and the remark- able myelonic eye (the eye in the brain) disappears. The number of gill-slits increases as the animal grows in size and its outer skin becomes tough and leather- like. Before saying anything further on the subject of degeneration, it seems desirable once more to direct. 0 WWl 0 Fig. 21. Fig. 22. Fig. 21.—Section through the eye ("surface eye") of a Water-beetleV-larva. All the cell
. The advancement of science. Occasional essays & addresses. Natural history. 38 DEGENERATION : shape, wliilst tlie cloacal chamber forms. The brain remains quite small and undeveloped, and the remark- able myelonic eye (the eye in the brain) disappears. The number of gill-slits increases as the animal grows in size and its outer skin becomes tough and leather- like. Before saying anything further on the subject of degeneration, it seems desirable once more to direct. 0 WWl 0 Fig. 21. Fig. 22. Fig. 21.—Section through the eye ("surface eye") of a Water-beetleV-larva. All the cells are seen to be in a row continuous with h, the cells of the outer- most skin or ectoderm, ^j, pigmented cells ; r, retinal cells connected at o with the optic nerve; g, transparent cells (forming a kind of "vitreous body"); I, cuticular lens. (From Gegenbaur's Elements of ComxMrative Anatomy, after Greuacher.) Fig. 22.—Section through the eye ("surface-eye") of a Marine Worm (Xeo- phanta). i, integument spreading over the front of the eye c ; I, cuticular lens ; h, cavity occupied by vitreous body ; xj, retinal cells ; h, pigment; o, optic nerve ; o', expansion of optic nerve. attention to the myelonic or cerebral eye which the Ascidian tadpole possesses in common with all Ver- tebrates. All other animals which have eyes develop the retina or sensitive part of the eye from their outer skin (see Figs. 21 and 22, and explanation). It is. 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 Lankester, E. Ray (Edwin Ray), Sir, 1847-1929. London and New York, Macmillan
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