. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 78 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Visual Organs. — The sensory portion of the eyes arises from the brain, and in the embryos of some vertebrates (elasmo- branchs, urodeles) optic areas can be recognized in the medullary. Fig. 84. Diagrams showing the inversion of the layers in the formation of the retina. In all the figures the nuclei are placed in the morphologically deeper ends of the cells. In A the brain (^) has been closed in, in B the optic vesicle {y) has reached the thickening for the lens (/), and on the r
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 78 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Visual Organs. — The sensory portion of the eyes arises from the brain, and in the embryos of some vertebrates (elasmo- branchs, urodeles) optic areas can be recognized in the medullary. Fig. 84. Diagrams showing the inversion of the layers in the formation of the retina. In all the figures the nuclei are placed in the morphologically deeper ends of the cells. In A the brain (^) has been closed in, in B the optic vesicle {y) has reached the thickening for the lens (/), and on the right side the vesicle is being con- verted into the double-walled cup with, as shown in C, a medial epithelial (?), and an outer retinal layer (^), the deeper face of which is turned towards the lens. plate before its involution. The accessor}' portions are furnished by ectoderm, mesothelium, and mesenchyme. A hollow outgrowth arises on either side of the primitive fore brain, and extends towards the skin. The distal portion expands into a globular optic vesicle, while the proximal por- tion retains its smaller size, and is known as the optic stalk. Thus the cavity of the vesicle is in connection with the ven- tricle of the thalamencephalon by means of the hollow stalk. The distal surface of each optic vesicle conies in contact with the ectoderm of the side of the head at the place where the lens is to form (see below), and with the formation of this structure the distal half of the optic vesicle becomes invaginated into the proximal part, thus partially obliterating the cavity of the vesicle, and converting the whole into a two-layered cup. The distal invaginated part of. 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 Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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