Journal of morphology . e fiber is the original basal end of the cell. Figs, i and 2indicate in a general way the points I have emphasized. The sense cells of theskin of Lumbricus, as well as the cephalopod eye, are indicated in the diagram.(For literature regarding these points, see Altmann, His, Merk, Rabl and others.) 4iS MALL. [Vol. VIII. With these ideas clearly in mind it is quite easy to understandwhy the optic nerve perforates the retina in the vertebrate eyeand passes directly to the brain in the invertebrate. They alsotell us why primitive eyes are composed of but a single layer ofce
Journal of morphology . e fiber is the original basal end of the cell. Figs, i and 2indicate in a general way the points I have emphasized. The sense cells of theskin of Lumbricus, as well as the cephalopod eye, are indicated in the diagram.(For literature regarding these points, see Altmann, His, Merk, Rabl and others.) 4iS MALL. [Vol. VIII. With these ideas clearly in mind it is quite easy to understandwhy the optic nerve perforates the retina in the vertebrate eyeand passes directly to the brain in the invertebrate. They alsotell us why primitive eyes are composed of but a single layer ofcells sending nerve fibers directly to the brain, and why in themore complex eyes a chain of cells is used for the same purpose. In the present study it has been my aim to follow the for-mation of the retina from the time the optic vesicle is wellmarked to the beginning of the optic nerve. Eycleshymer ^ hasalready shown us that in Amblystoma and Necturus the opticcups are well marked long before the neural canal begins to. Fig. 3. — Section through the eye of Amblystoma ( mm. long). X 66 a drawing by Mr. Eycleshymer. O. V., optic vesicle. close by pigmented areas which are slightly cup-shaped, and aremade up of a single layer of cylindrical cells with the pigmentat either end of them. As the areas grow more and more toform cups the mitoses are more numerous in them than in thesurrounding ectoderm which is to form the neural canal. Thepigment gradually becomes more and more diffuse and servesto a great extent in following the differentiation of the the later stages of development the walls of thevesicles become thinner, so that just before they invaginate to 1 Eycleshymer, Journal of Morphology, Vol. VIII. No. 2.] HISTOGENESIS OF THE RETINA. 419 form the optic cups they consist of but a single layer ofelongated cells with their nuclei located in their peripheralends. A continued dispersion and migration of the pigmenthas also taken place, and is now m
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