The similarity between these conspicuous layers of lightly-staining cells in Ammocœtes and in crustaceans is remarkably close, and in both cases observers have found the same difficulty in interpreting their meaning. In each case one group of observers looks upon them as ganglion-cells, the other as supporting structures. Thus in the lamprey, Müller considers them to belong to the supporting elements, while Langerhans and Kohl describe them as a double layer of ganglion-cells. In the crustacean, Berger in Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as supporting elements, while Viallanes in Palinurus considers them to be true ganglionic cells.
Whatever the final interpretation of these cells may prove to be, we may, it seems to me, represent an ideal compound retina of the crustacean type by combining the investigations of Berger, Claus, Reichenbach, and Parker in the following figure.
Fig. 42.—Ideal Diagram of the Layers in a Crustacean Eye.
The retina is divided into an epithelial part, C (the layer of retinular cells and rhabdomes), and a neurodermal or cerebral part, which is formed of, A, the ganglion of the optic nerve, and, B, the ganglion of the retina. 1, optic nerve fibres which cross at their entrance into the retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer of crystalline cones; 9, cornea.
The comparison of this figure (Fig. [42]) with that of the Petromyzon retina (Fig. [41]) shows how great is the similarity of the latter with the arthropod type, and how the very points in which it deviates from the recognized vertebrate type are explainable by comparison with that of the arthropod. The most striking difference between the retinas in the two figures is that the layer of terminal nerve fibres (5, Fig. [42]), which, after all, are only the elongated terminations of the retinal cells belonging to Parker's neurones of the first order, is very much longer than in Petromyzon or in any vertebrate, for the external molecular layer (6, Fig. [41]) (Müller's layer of Nervenansätze) is very short and inconspicuous (in Fig. [41] it is drawn too thick).
Turning from the retina to the fibres of the optic nerve we again find a remarkable resemblance, for in Ammocœtes, as pointed out by Langerhans and carefully figured by Kohl, a crossing of the fibres of the optic nerve occurs as the nerve leaves the retina, just as is so universally the case in all compound retinas. To this crossing Kohl has given the name chiasma nervi optici, in distinction to the cerebral chiasma, which he calls chiasma nervorum opticorum. Further, we find that even this latter chiasma is well represented in the arthropod brain; thus Bellonci in Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a true optic chiasma, the only difference in opinion being, whether the crossing of the optic nerves is complete or not. Especially instructive are Bellonci's figures and description. He describes the brain of Sphæroma as composed of three segments—a superior segment, the cerebrum proper, a middle segment, and an inferior segment; the optic fibres, as is seen in Fig. [39], after crossing, pass direct into the middle segment, in the ganglia of which they terminate. From this segment also arises the nerve to the first antenna of that side—i.e. the olfactory nerve. The optic part, then, of this middle segment is clearly the brain portion of the optic ganglionic apparatus, and may be called the optic lobes, in contradistinction to the peripheral part, which is usually called the optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as already mentioned. These optic lobes are therefore homologous with the optic lobes of the vertebrate brain.
The resemblance throughout is so striking as to force one to the conclusion that the retina of the vertebrate eye is a compound retina, composed of a retina and retinal ganglion of the type found in arthropods. From this it follows that the development of the vertebrate retina ought to show the formation of (1) an optic plate formed from the peripheral epidermis and not from the brain; (2) a part of the brain closely attached to this optic plate forming the retinal ganglion, which remains at the surface when the rest of the optic ganglion withdraws; (3) an optic nerve formed in consequence of this withdrawal, as the connection between the retinal and cerebral parts of the optic ganglion.
This appears to me exactly what the developmental process does show according to Götte's investigations. He asserts that the retina arises from an optic plate, being the optical portion of his 'Sinnes-platte.' At an early stage this is separated by a furrow (Furche) from the general mass of epidermal cells which ultimately form the brain. This separation then vanishes, and the retina and brain-mass become inextricably united into a mass of cells, which are still situated at the surface. By the closure of the cephalic plate and the withdrawal of the brain away from the surface, a retinal mass of cells is left at the surface connected with the tubular central nervous system by the hollow optic diverticulum or primary optic vesicle. If we regard only the retinal and nervous elements, and for the moment pay no attention to the existence of the tube, Götte's observation that the true retina has been formed from the optic plate (Sinnes-platte) to which the retinal portion of the brain (retinal ganglion) has become firmly fixed, and that then the optic nerve has been formed by the withdrawal of the rest of the brain (optic lobes), is word for word applicable to the description of the development of the compound retina of the arthropod eye, as has been already stated.
The Significance of the Optic Diverticula.