The development of the human body; a manual of human embryology . ion cellspassing into it on their way to the optic nerve. Thethicker layer similarly suggests a comparison with themantle layer of the cord and brain, and in embryos of 38mm. it becomes differentiated into two secondary layers(Fig. 258), that nearest the pigment layer (r) consistingof smaller and more deeply staining nuclei, probablyrepresenting the rod and cone and bipolar cells of the adultretina, while the inner layer, that nearest the marginalvelum, has larger nuclei and is presumably composed ofthe ganglion cells. Little is
The development of the human body; a manual of human embryology . ion cellspassing into it on their way to the optic nerve. Thethicker layer similarly suggests a comparison with themantle layer of the cord and brain, and in embryos of 38mm. it becomes differentiated into two secondary layers(Fig. 258), that nearest the pigment layer (r) consistingof smaller and more deeply staining nuclei, probablyrepresenting the rod and cone and bipolar cells of the adultretina, while the inner layer, that nearest the marginalvelum, has larger nuclei and is presumably composed ofthe ganglion cells. Little is as yet known concerning the further differentia-tion of the nervous elements of the human retina, but the THE RETINA. 487 history of some of them has been traced in the cat, inwhich, as in other mammals, the histogenetic processestake place at a relatively later period than in man. Of thehistogenesis of the inner layer the information is ratherscant, but it may be stated that the ganglion cells are theearliest of all the elements of the retina to become recog-. 00 0o 00 o %6V o Fig. 258.—Portion of a Transverse .Section of the Retina of a New-born , Chorioid coat; g, ganglion-cell layer; r, outer layer of nuclei; p, pig-ment layer.—(Falchi.) nizable. The rod and cone cells, when first distinguish-able, are unipolar cells (Fig. 259, a and c), their singleprocesses extending outward from the cell-bodies to theexternal limiting membrane which bounds the outer sur-face of the retinal layer. Even at an early stage the conecells (a) are distinguishable from the rod cells (c) by their 488 THE DEVELOPMENT OF THE HUMAN BODY. more decided reaction to silver salts, and at first bothkinds of cells are scattered throughout the thickness ofthe layer from which they arise. Later, a fine processgrows out from the inner end of each cell, which thus as-sumes a bipolar form (Fig. 259, b and d), and, later still,the cells gradually migrate toward the external limiting
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
