The Origin of Vertebrates

by Walter Holbrook Gaskell

B. A neurodermal (cerebral) part which forms the rest of the retina.

Further, Müller points out that the neuroderm gives origin throughout the central nervous system to two totally different structures, on the one hand to the true nervous elements, on the other to a system of supporting cells and fibres which cannot be classed as connective tissue, for they do not arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In the retina he recognizes two distinct groups of such supporting structures—(1) a system of radial fibres with well-marked elongated nuclei, which extend between the two limiting layers, and form at their outer ends a membrane-like expansion which was originally the outer limit of the retina, but becomes afterwards co-terminous with the membrana limitans externa, owing to the piercing through it of the external limbs of the rods. This system, which is known by the name of the radial Müllerian fibres (shown on the left-hand side of Fig. [41]), has no connection with (2) the spongioblasts and neurospongium, which form a framework of neuroglia, in which the terminations of the optic ganglion and of the retinal ganglion ramify to form the molecular layers.

It is evident from Fig. [41] that the retina of Ammocœtes and Petromyzon differs in a striking manner from the typical vertebrate retina. The epithelial part (C) remains the same—viz. the visual rods, the external limiting membrane, and the external nuclear layer; but the cerebral part, the retinal ganglion (A and B), is remarkably different. It is true, it consists in the main of the small-celled mass known as the inner nuclear layer, and of the reticulated tissue or 'neuropil' known as the inner molecular layer, just as in all other compound retinal eyes; but neither the ganglion cell-layer nor the optic fibre-layer is clearly defined as separate from this molecular layer; on the contrary, it is matter of dispute as to what cells represent the ganglionic layer of higher vertebrates, and the optic fibres do not form a distinct innermost layer, but pass into the inner molecular layer at its junction with the inner nuclear layer. A comparison of this innermost part of the retina (A, Fig. 41), with the corresponding part in Berger's picture of Musca (n.l.o.g., Fig. [38]), shows a most striking similarity between the two. In both cases the fibres of the optic nerve (O.n., Fig. [38]) which cross at their entrance pass into the 'neuropil' of this part of the retinal ganglion, and are connected probably (though that is not proved in either case) with the cells of the ganglionic layer. In both cases we find two well-marked parallel rows of cells in this part of the retina, of which one, the innermost, is composed in Ammocœtes of large ganglion-cells, and the other mainly of smaller, deeper staining cells apparently supporting in function. Similarly, also, in Branchipus, as I conclude from my own observations as well as from those of Berger and Claus, the ganglionic layer is composed partly of true ganglion-cells and partly of supporting cells arranged in a distinct layer. This part, then, of the retina of Ammocœtes is remarkably like that of a typical arthropod retina, and forms that part of the retinal ganglion which may be called the ganglion of the optic nerve.

Next comes the ganglion of the retina (B, Fig. [41]) (Parker's first optic ganglion), the cells of which form the small bipolar granule-cells of the inner nuclear layer; granule-cells arranged in rows just as they are shown in Claus' picture of the same layer in the retina of Branchipus (Fig. [40]), just as they are found in the cortical layers of the optic ganglion of the pineal eye (ganglion habenulæ), in the optic lobes and other parts of the Ammocœtes brain, or in the cortical layers of the optic ganglia of all arthropods.

Between this small-celled nuclear layer (4, Fig. [41]) and the layer of nuclei of the visual rod cells (7, Fig. [41]) (the external nuclear layer), we find in the eye of Ammocœtes and Petromyzon two well-marked rows of cells of a most striking character—viz. the two remarkably regular rows of large epithelial-like cells with large conspicuous nuclei, which give the appearance of two opposing rows of limiting epithelium (5, Fig. [41]), already mentioned in connection with the researches of Langerhans and W. Müller. Here, then, is a striking peculiarity of the retina of the lamprey, and according to Müller the obliteration of these two layers can be traced as we pass upwards in the vertebrate kingdom. Among fishes, they are especially well seen in the perch; in the higher vertebrates the whole layer is only a rudiment represented, he thinks, by the simple layer of round cells which lies close against the inner surface of the layer of terminal fibres (Nervenansätze), and is especially evident in birds and reptiles. In man and the higher mammals they are probably represented by the horizontal cells of the outer part of the inner nuclear layer.

Seeing, then, that they are most evident in Ammocœtes, and become less and less marked in the higher vertebrates, it is clear that their origin cannot be sought among the animals higher in the scale than Ammocœtes, but must, therefore, be searched for in the opposite direction.

Müller describes them as forming a very conspicuous landmark in the embryology of the retina, dividing it distinctly into two parts, an outer thinner, and an inner somewhat thicker part, the zone formed by them standing out conspicuously on account of the size and regularity of the cells and their lighter appearance when stained. Thus in his description of the retina of an Ammocœtes 95 mm. in length, he says, "The layer of pale tangentially elongated cells formed a double layer and produced the appearance of a pale, very characteristic zone between the outer and inner parts of the retina."

Let us now turn to the retina of the crustacean and see whether there is any evidence there that the retina is divisible into an outer and inner part, separated by a zone of characteristically pale staining cells with conspicuous nuclei. The most elaborate description of the development of the retina of Astacus is given by Reichenbach, according to whom the earliest sign of the formation of the retina is an ectodermic involution (Augen-einstülpung), which soon closes, so that the retinal area appears as a thickening. In close contiguity to this thickening, the thickening of the optic ganglion arises, so that that part of the optic ganglion which will form the retinal ganglion fuses with the thickened optic plate and forms a single mass of tissue. Later on a fold (Augen-falte) appears in this mass of tissue, in consequence of which it becomes divided into two parts. The lining walls of this fold form a double row of cells, the nuclei of which are most conspicuous because they are larger and lighter in colour than the surrounding nuclei, so that by this fold the retina is divided into an outer and an inner wall, the line of demarcation being conspicuous by reason of these two rows of large, lightly-staining nuclei.

Reichenbach is unable to say that this secondary fold is coincident with the primary involution, and that therefore the junction between the two rows of large pale nuclei is the line of junction between the retinal ganglion and the retina proper, because all sign of the primary involution is lost before the secondary fold appears.

Parker compares the appearances in the lobster with Reichenbach's description in the crayfish, and says that he finds only a thickening, no primary involution; at the same time he expressly states that in the very early stages his material was deficient, and that he had not grounds sufficient to warrant the statement that no involution occurs. He also finds that in the lobster the ganglionic tissue which arises by proliferation is divided into an outer and inner part; the separation is effected by a band of large, lightly-staining nuclei, which, in position and structure, resemble the band figured by Reichenbach. According to Parker, then, the line of separation indicated in the development by Reichenbach's outer and inner walls is not the line of junction between the retina and the retinal ganglion, as Reichenbach was inclined to think, but rather a separation of two rows of large ganglion-cells belonging to the retinal ganglion.