{93}The comparison of the arthropod compound retina with that of the vertebrate shows, as one would expect upon the theory of the origin of vertebrates put forward in this book, that the latter retina is built up of two ganglia, as in the more primitive less specialized crustacean forms. The modern description of the vertebrate retina, based upon the Golgi method of staining, is exactly Parker's description of the simpler form of crustacean retina in which the 'neuropil' of the first ganglion is represented by the external molecular layer, and that of the second ganglion by the internal molecular layer; the three sets of neurones being, according to Parker's terminology:-- 1. The neurones of the first order--viz. the visual cells--the nuclei of which form the external nuclear layer, and their long attenuated processes form synapses in the external molecular layer with 2. The neurones of the second order, the cells of which form the internal nuclear layer, and their processes form synapses in the internal molecular layer with 3. The neurones of the third order, the cells of which form the ganglionic layer and their neuraxons constitute the fibres of the optic nerve which end in the optic lobes of the brain. Strictly speaking, of course, the visual cells with their elongated processes have no right to be called neurones: I only use Parker's phraseology in order to show how closely the two retinas agree even to the formation of synapses between the fine drawn-out processes of the visual cells and the neurones of the ganglion of the retina. THE RETINA OF THE LATERAL EYE OF AMMOCOETES. As in the case of all other organs, it follows that if we are dealing here with a true genetic relationship, then the lower we go in the vertebrate kingdom the more nearly ought the structure of the retina to approach the arthropod type. It is therefore a matter of intense interest to determine the nature of the retina in Ammocoetes in order to see whether it differs from that of the higher vertebrates, and if so, whether such differences are explicable by reference to the structure of the arthropod eye. Before describing the structure of this retina it is necessary to clear away a remarkable misconception, shared among others by {94}Balfour, that this eye is an aborted eye, and that it cannot be considered as a primitive type. Thus Balfour says: "Considering the degraded character of the Ammocoete eye, evidence derived from its structure must be received with caution," and later on, "the most interesting cases of partial degeneration are those of Myxine and the Ammocoete. The development of such aborted eyes has as yet been studied only in the Ammocoete, in which it resembles in most important features that of other Vertebrata."