The Origin of Vertebrates

by Walter Holbrook Gaskell

(2) The layer of nerve-bundles.

(3) The nuclear layer.

(4) The molecular layer.

(5) The ganglion cell layer.

Berger passes under review the structure and arrangement of the optic ganglion in a large number of different groups of arthropods, and concludes that in all cases one part of the optic ganglion is always closely attached to the visual end-cells, and this combination he calls the retina. On the other hand, the nerve-fibres which connect the peripheral part of the optic ganglion with the brain, the so-called optic nerve, are by no means homologous in the different groups; for in some cases, as in many of the stalk-eyed crustaceans, the whole optic ganglion is at the periphery, while in others, as in the Diptera, only the retinal ganglion is at the periphery, and the nerve-stalk connects this with the rest of the optic ganglion, the latter being fused with the main brain-mass. In the Diptera, in fact, according to Berger, the optic nerve and retina are most nearly comparable to those of the vertebrate. For this reason I give Berger's picture of the retina of Musca (Fig. [38]), in order to show the arrangement there of the retinal layers.

Fig. 39.—The Brain of Sphæroma serratum. (After Bellonci.)

Ant. I. and Ant. II., nerves to 1st and 2nd antennæ. f.br.r., terminal fibre-layer of retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n., optic nerve-fibres forming an optic chiasma.

In Branchipus and other primitive Crustacea, Berger also finds the same retinal layers, but is unable to distinguish in the brain the rest of the optic ganglion. Judging from Berger's description of Branchipus, and Bellonci's of Sphæroma, it would almost appear as though the cerebral part of the retina in the higher forms originated from two ganglionic enlargements, an external and internal enlargement, as Bellonci calls them. The external ganglion (Op. g. I., Fig. [39]) may be called the ganglion of the retina, the cells of which form the nuclear layer of the higher forms, and the internal ganglion (Op. g. II., Fig. [39]), from which the optic nerve-fibres to the brain arise, may therefore be called the ganglion of the optic nerve. Bellonci describes how in this latter ganglion cells are found very different to the small ones of the external ganglion or ganglion of the retina. So also in Branchipus, judging from the pictures of Berger, Claus, and from my own observations (cf. Fig. [46], in which the double nature of the retinal ganglion is indicated), the peripheral part of the optic ganglion—i.e. the retinal ganglion—may be spoken of as composed of two ganglia. The external of these is clearly the ganglion of the retina; its cells form the nuclear layer, the striking character of which, and close resemblance to the corresponding layer in vertebrates, is shown by Claus' picture, which I reproduce (Fig. [40]). The internal ganglion with which the optic nerve is in connection contains large ganglion cells, which, together with smaller ones, form the ganglionic layer of Berger.

The most recent observations of the structure of the compound retina of the crustacean eye are those of Parker, who, by the use of the methylene blue method, and Golgi's method of staining, has been able to follow out the structure of the compound retina in the arthropod on the same lines as had already been done for the vertebrate. These two methods have led to the conclusion that the arthropod central nervous system and the vertebrate central nervous system are built up in the same manner—viz. by means of a series of ganglia connected together, each ganglion being composed of nerve-cells, nerve-fibres, and a fine reticulated substance called by Leydig in arthropods 'Punktsubstanz,' and known in vertebrates and in invertebrates at the present time as 'neuropil.' A further analysis resolves the whole system into a combination of groups of neurones, the cells and fibres of which form the cells and fibres of the ganglia, while their dendritic connections with the terminations of other neurones, together with the neuroglia-cells form the 'neuropil.' As is natural to expect, that part of the central nervous system which helps to form the compound retina is built up in the same manner as the rest of the central nervous system.