Fig. 470. Vertical section through retina of a five and one-half months' human embryo
Modified from Lange.
The bipolar cells (Fig. 468, e), which with their processes constitute the middle or second optic neurone, also develop from cells of the nuclear layer and are probably bipolar at the time that the rod and cone cells are in the unipolar condition. Reference to the two bipolar cells shown in Fig. 468, e, ey shows that at this stage in their development their outwardly directed processes extend to the outer limiting membrane. These processes must either actually shorten or else fail to grow in length proportionately as the retina increases in thickness, for in the mature retina they end in relation with the centrally (inwardly) directed processes (axones) of the rod and cone cells. According as they are in relation with rod cells or cone cells, they are known as rod bipolars or cone bipolars. The retinal layer in which the axones of the rod and cone cells and the dendrites of the rod and cone bipolars intermingle is the outer, molecular layer of the adult retina. It is first distinctly recognizable as a molecular layer about the end of the fifth month (Fig. 470).
The development of the outer molecular layer separates the originally single nuclear layer into two layers, an outer composed of the nuclei of the rod and cone cells and an inner composed of the nucleated bodies of the rod and cone bipolars, of the horizontal cells (Fig. 468, g) and of the amacrine cells (Fig. 468, / and f), all of which can be recognized in Golgi specimens by the end of the seyegth month. The rod and cone bipolars and probably most of the other cells of the inner nuclear layer send their axones centrally to lie in contact with the dendrites and bodies of the ganglion cells.
With the development of the cells of the inner nuclear layer and their processes, there differentiates the inner molecular layer which separates the inner nuclear layer and the layer of ganglion cells. It consists mainly of ramifications of the dendrites and axones of cells the bodies of which lie in the inner nuclear layer and in the layer of ganglion cells. (Fig. 470.)
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