Fig. 30.—Diagram of Formation of an Upright Compound Retina.
ABCD, as in Fig. [28]. Op. g. I. and Op. g. II., two optic ganglia which combine to form the retinal ganglion, Rt. g.
When, therefore, a compound retina is formed by the amalgamation of the ectodermal part—the retinal cells proper—with the neurodermic part—to which the name 'retinal ganglion' may be given,—such a retina consists of neuropil substance and nerve-cells, as well as the retinal end-cells. In all such compound retinas, the retinal ganglion is not single, but two optic ganglia at least are included in it, so that there are two sets of nerve-cells and two synapses are always formed; one between the retinal end-cells and the neurones of the first optic ganglion, which may be called the ganglion of the retina, the other between the first and second ganglia, which, seeing that the neuraxons of its cells form the optic nerve, may be called the ganglion of the optic nerve. The 'neuropil' formed by these synapses forms the molecular layers of the compound retina, and the cells themselves form the nuclear layers. Thus an upright compound retina, formed in the same way as the upright simple retina, would be illustrated by Fig. [30].
Further, in precisely the same way as in the case of the simple retina, such a compound retina may be upright or inverted. Thus, in the lateral eyes of crustaceans and insects, a compound retina of this kind is formed, which is upright; while in the vertebrates the compound retina of the lateral eyes is inverted.
The compound retina of vertebrates is usually described as composed of a series of layers, which may be analyzed into their several components as follows:—
| Layer of rods and cones External nuclear layer External molecular layer Internal nuclear layer Internal molecular layer Optic nerve-cell layer Layer of optic nerve fibres |  | retina proper |  | Ectodermic part | ||
| ganglion of retina |  | retinal ganglion | | neurodermic part | |
| ganglion of optic nerve | |||||
| Layer of rods and cones | retina proper | Ectodermic part | |
| External nuclear layer | |||
| External molecular layer | |||
| ganglion of retina | retinal ganglion | neurodermic part | |
| Internal nuclear layer | |||
| Internal molecular layer | |||
| ganglion of optic nerve | |||
| Optic nerve-cell layer | |||
| Layer of optic nerve fibres | |||
The difference between the development of these two types of eye—those with a simple retina and those with a compound retina—has led, in the most natural manner, to the conception that the retina is developed, in the higher animals, sometimes from the cells of the peripheral epidermis, sometimes from the tissue of the brain—two modes of development termed by Balfour 'peripheral' and 'cerebral.' An historical survey of the question shows most conclusively that all investigators are agreed in ascribing the origin of the simple retina to the peripheral method of development, the retina being formed from the hypodermal cells by a process of invagination, while the cerebral type of development has been described only in the development of the compound retina. The natural conclusion from this fact is that, in watching the development of the compound retina, it is more difficult to differentiate the layers formed from the epidermal retinal cells and those formed from the epidermal optic ganglion-cells, than in the case of the simple retina, where the latter cells withdraw entirely from the surface. This is the conclusion to which Patten has come, and, indeed, judging from the text-book of Korschelt and Heider, it is the generally received opinion of the day that, as far as the Appendiculata are concerned, the retina, in the true sense—the retinal end-cells, with their cuticular rods,—is formed, in all cases, from the peripheral cells of the hypodermal layer, the cuticular rods being modifications of the general cuticular surface of the body. The apparent cerebral development of the crustacean retina, as quoted from Bobretsky by Balfour, is therefore in reality the development of the retinal ganglion, and not of the retina proper.
There is, I imagine, a universal belief that the natural mode of origin of a sense-organ, such as the eye, must always have been from the cells forming the external surface of the animal, and that direct origin from the central nervous system is a priori most improbable. It is, therefore, a matter of satisfaction to find that the evidence for the latter origin has universally broken down, with the single exception of the eyes of vertebrates and their degenerated allies; a fact which points strongly to the probability that a reconsideration of the evidence upon which the present teaching of the origin of the vertebrate eye is based will show that here, too, a confusion has arisen between that part formed from the epidermal surface and that from the optic ganglion.