The development of the human body : a manual of human embryology / by J. Playfair McMurrich.

  • McMurrich J. Playfair (James Playfair), 1859-1939.
Date:
1910
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Licence: In copyright

Credit: The development of the human body : a manual of human embryology / by J. Playfair McMurrich. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    (r) consisting- of smaller and more deeply staining nuclei, probably representing the rod and cone and bipojar^^ of the adult retina, while the inner layer, that nearest the mar- ginal velum, has larger nuclei and is presumably composed of the ganglion cells. Little is as yet known concerning the further differentia- tion of the nervous elements of the human retina, but the history of some of them has been traced in the cat, in which, as in other mammals, the histogenetic processes take place at a relatively later period than in man. Of the histogenesis of the inner layer the information is rather scant, but it may be stated that the ganglion cells are the earliest of all the elements of the retina to become recognizable. The rod and cone cells, when first distinguishable, are unipolar cells (Fig. 266, a and c), their single processes extending out- ward froni_the^ell2bodies to the external limiting membrane which bounds the outer surface of the retinal layer. Even at an early stage the cone cells (a) are distingiushaWe J^^^^^ the rod cells (c) by their more decided reaction to silver salts, and at first both kinds of cells are scattered throughout the thickness of the layer from which they arise. Later, a fine process grows out from the inner end of„eadiXi&ll,.jyiiidi. thus assumes a bipolar form (Fig. 266, b and d), and, later stij], the cells gradually migrate toward the external Hniitin^ membrane, beneath which they form a definite layer-iii the .adiilt. In the meantime there appears. opposite the outer end of each cell a rounded eminence projecting from the outer surface of the external limiting membrane into the pigment layer. Thjeeipinences over the cone cells are larger than those over the rod cells, and later, as both increase in length, they become recognizable by their shape as the rods and cones. The bipolar cells are not easily distinguishable in the early stages of their differentiation from the other cells