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. {73}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 } retina proper } Ectodermic part External molecular layer }} Internal nuclear layer } ganglion of retina } Internal molecular layer }} } retinal } neurodermic Optic nerve-cell layer } ganglion of optic nerve} ganglion} part Layer of optic nerve } fibres