The Retina. At first the two walls of the optic cup do not greatly differ in thickness. On the third day the outer or posterior becomes much thinner than the inner or anterior, and by the middle of the fourth day is reduced to a single layer of flattened cells ([fig. 289], p.Ch). At about the 80th hour its cells commence to receive a deposit of pigment, and eventually form the so-called pigmentary epithelium of the choroid; from them no part of the true retina (or no other part of the retina, if the pigment-layer in question be supposed to belong more truly to the retina than to the choroid) is derived.
On the fourth day, the inner (anterior) wall of the optic cup ([fig. 289], R) has a perfectly uniform structure, being composed of elongated somewhat spindle-shaped cells, with distinct nuclei. On its external (posterior) surface a distinct cuticular membrane, the membrana limitans externa, early appears.
As the wall increases in thickness, its cells multiply rapidly, so that it soon becomes several cells thick: each cell being however probably continued through the whole thickness of the layer. The wall at this stage corresponds closely in its structure with the brain, of which it may properly be looked upon as part. According to the usual view, which is not however fully supported by the development, the retina becomes divided in the subsequent growth into (1) an outer part, corresponding morphologically to the epithelial lining of the cerebrospinal canal, composed of what may be called the visual cells of the eye, i.e. the cells forming the outer granular (nuclear) layer and the rods and cones attached to them; and (2) an inner portion consisting of the inner granular (nuclear) layer, the inner molecular layer, the ganglionic layer and the layer of nerve-fibres corresponding morphologically to the walls of the brain. According to Löwe, however, only the outer limbs of the rods and cones, which he holds to be metamorphosed cells, correspond to the epithelial layer of the brain.
The actual development of the retina is not thoroughly understood. According to the usual statements (Kölliker, No. [298], p. 693) the layer of ganglion cells and the inner molecular layer are first differentiated, while the remaining cells give rise to the rest of the retina proper, and are bounded externally by the membrana limitans externa. On the inner side of the ganglionic layer the stratum of nerve-fibres is also very early established. The rods and cones are formed as prolongations (Kölliker, Babuchin), or cuticularizations (Schultze, W. Müller) of the cells which eventually form the outer granular layer. The layer of cells external to the molecular layer is not divided till comparatively late into the inner and outer granular (nuclear) layers, and the interposed outer molecular layer.
Löwe’s account of the development of the retina in the Rabbit is in many points different from the above. He finds that three stages in the differentiation of the layers of the retina may be distinguished.
In the first stage, in an embryo of four or five millimetres, the following layers are present, commencing at the outer side, adjoining the external wall of the secondary optic cup.
(1) A membrane, which does not however, as usually believed, become the membrana limitans externa.
(2) A layer of clear elements, derived from metamorphosed cells, constituting the outer limbs of the rods and cones.
(3) A layer of dark rounded elements.
(4) An indistinctly striated layer, the future layer of nerve-fibres.
The third of these layers gives rise to all the eventual strata of the retina proper, except the outer limbs of the rods and cones.
In the next stage, when the embryo has reached a length of 2 cm., this layer becomes divided into three strata: viz. an outer and inner layer of dark elements and a middle one of clearer elements. The two inner of these layers become respectively the inner molecular layer and the layer of ganglion cells, while the outer layer gives rise to the parts of the retina external to the inner molecular layer.
In the newly born animal the outer darker layer of the previous stage has become considerably subdivided. Its outermost part forms a stratum of darkly coloured elements, which develop into the inner limbs of the rods and cones. It is bounded internally by a membrane—the true membrana elastica externa. The part of the layer within this is soon divided into the outer and inner granular layers, separated from each other by the delicate outer molecular layer. Thus, shortly after birth, all the layers of the retina are established in the Rabbit. It is important to notice that, according to Löwe’s views, the outer and inner limbs of the rods and cones are metamorphosed cells. The outer limbs at first form a continuous layer, in which separate elements cannot be recognised.
At a very early period there appears a membrane on the side of the retina adjoining the vitreous humour. This membrane is the hyaloid membrane. The investigations of Kessler and myself lead to the conclusion that it may be formed at a time when there is no trace of mesoblastic structures in the cavity of the vitreous humour, and that it is therefore necessarily developed as a cuticular deposit of the cells of the optic cup. Lieberkühn, Arnold, Löwe, and other authors regard it however as a mesoblastic product; and Kölliker believes that a primitive membrane is developed from the cells of the optic cup, and that a true hyaloid membrane is developed much later as a product of the mesoblast.