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posed part of the body. Why it occurs in spots we know not, any more than we know why freckles come in spots. It may occur anywhere, but usually near the most important ganglion—the cephalic—and therefore in the head. Thus far we have a simple mechanism for perception of light, but not yet of objects, for we have not yet an image-forming instrument. Such a pigmented group of modified cells with specialized nerve
DIAGRAM REPRESENTING THE DIFFERENT STAGES IN THE EVOLUTION OF THE
INVERTEBRATE EYE. a b c, eye-spots, no image; d, pin-hole image; e, simple lens-image; f, compound lens-image; r, retina; o 11, optic nerve; v, vitreous humor; 1, lens; cor, cornea.
attached is called an eye-spot. It is not yet an eye proper. Such eye-spots occur in very many lowest animals (Fig. 148, a).
The next step is a slight saucer-shaped depression of the modified spot with a slightly raised rim about it, as shown in b. This condition is found in the solen. In these headless mollusks the eye-spots are strung all along the edge of the mantle. Next the depression becomes deeper and cup-shaped, and the rim higher. This is found in the limpet (Patella), c. This is an improvement in so far as the light sensitive spot is protected and the impression is stronger by reverberation within the hollow. The eye-spot in these are on the head.
Thus far we have eye-spots, not true eyes-an organ perceiving light but not objects. The next step is found in the nautilus (d). In this case the raised margin of the hollow is drawn together until only a pin-holeopening remains. Now for the first time we have an inverted image on the concave retina, but as yet only a pin-hole-image. We have already seen (page 19), the imperfection of such an image, and therefore the imperfect and blurred perception of objects.
The next step is found in the trochus and many other gasteropods—for example, the snail (e). The pinhole-opening is closed although the point of closing remains transparent, and the hollow is filled with transparent refractive substance, which may be likened to the vitreous humor, although often called the lens. Here, then, we have a concave retina with bacillary layer—a refracting humor-a transparent cornea which may also be called a pupil. In this case we have an image formed by a lensa lens-image--and therefore a far more perfect perception of objects.
The next and final step is found in the squid (f). In this animal that portion of the epithelial surface which covers the front of the eye, by a cuticular ingrowth forms a lens; and by folds of the epidermal surface lids are also formed. In fact, nearly all the parts of the vertebrate eye are found in these. therefore, we have, as in the vertebrate eye, not only a lens-image, but a compound lens-image.
That we have really given a true outline of the evo
In this case,
lution of the invertebrate eye is shown by the fact that these very steps are taken in the embryonic development of the eye of the squid. First a spot becomes depressed with a raised rim about it. Then the riin rises so as to make a deep hollow. Then the edges of the deep concave closes until only a pin-hole-opening is left. Then the opening closes and the hollow becomes a vesicle filled with refractive matter—the vitreous humor. Then a cuticular ingrowth from the central point of the surface forms the lens, and last iris and lids are added.
We have given only the barest outlines of the most important steps. There are many intermediate steps not mentioned. We have said nothing, also, of the compound eye of insects and crustaceans, because these are wholly peculiar and out of the line of the gradual evolution of this organ. Thus far we find nothing like an optic chiasm, nor fovea, and almost certainly the phenomena of binocular vision have not yet appeared. From the manner in which the fibers terminate it is evident that there can be no blind spot.
1. The Vertebrate Eye. There are two great and essential differences in structure and mode of formation between the invertebrate and the vertebrate eye. 1. In the invertebrate eye the nerve fibers terminate directly in the inner ends of the nerve terminals or retinal rods, and the farther ends of these look forward and outward to receive the light. This seems the most natural mode, and is universal in the nerve terminals of all other senses in all animals. But in man and in all vertebrates the fibers of the optic nerve turn back on themselves and terminate in the outer ends of the rods and cones, and the extreme ends of these latter look backward and inward (Fig. 24, page 53). This is wholly exceptional among nerve terminals. 2. It is seen that in invertebrates the whole eye—both retina and lens, both receiving plate and image-making instrument–is made from the epidermal epithelium. But in man and in all vertebrates embryology shows that the eye is formed partly by infolding of the epidermal epithelium and partly by the outfolding of the brain vesicle and its epithelial lining ; the image-making instrument is made from the epiderm, and the receptive plate, the retina, from the brain.
The steps of the whole process is briefly as follows: The brain in very early embryonic condition consists of three hollow vesicles in linear series. From the anterior one of these—the thalamus and cerebrum-originates the retinal part of the eye by an outfolding on each side (Fig. 149, A). This outfolding continues until the subordinate vesicle—the optic vesicle, which is to become the retina-is connected with the brain vesicle only by a narrow neck which becomes the optic nerve (Fig. 149, B). In the meantime there has commenced a corresponding infolding from the epidermal surface to form the lens, and this is finally separated from its epidermal connection (B). Next, the optic vesicle is infolded on its anterior surface like a double nightcap, so that its middle part becomes widely separated from the lens (C). Of the two layers of the optic vesicle thus formed the anterior one, r, becomes the retina, and the posterior one, ch, the choroid. Now the whole interior of the brain vesicle, and therefore of the optic vesicle, is lined with a continuous pavement of epithelial cells. Therefore, in the two layers r and ch of the concave retina, the posterior surface of the anterior layer, and the anterior surface of the posterior layer are epithelial (D). Now, as already said, the anterior layer becomes the retina, and therefore this posterior epithelial part becomes the bacillary layer. Similarly the anterior or epithelial part of the posterior layer becomes
DIAGRAM REPRESENTING DIFFERENT STAGES IN THE DEVELOPMENT OF THE
VERTEBRATE EYE. CV, cerebral vesicle; 0 V, optic vesicle; r, retina; ch, choroid; 6, bacillary layer; f, fibrous layer; 1, lens.
the pigmentary layer of the choroid. It is for this reason that the choroid is sometimes called a part of the retina. Like the retina, its origin is cerebral. From this mode of origin it is evident that the bacillary layer is the most posterior part of the retina, instead of the most anterior as in invertebrates : and that the fibres turn back to terminate in the rods and cones. It is