102 THE ORIGIN OF VERTEBRATES 



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 tjhe 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, Gotte's obser- 

 vation 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. 



The origin of the retina from an optic epidermal plate in verte- 

 brates, as in all other animals, brings the cephalic eyes of all animals 

 into the same category, and leaves the vertebrate eye no longer in an 

 isolated and unnatural position. In one point the retina of the verte- 

 brate eye differs from that of a compound retina of an invertebrate ; 

 in the former, a striking supporting tissue exists, known as Midler's 

 fibres, which is absent in the latter. This difference of structure is 

 closely associated with another of the same character as in the central 

 nervous system, viz. the apparent development of the nervous part from 

 a tube. We see, in fact, that the retinal and nervous arrangements of 

 the vertebrate eye are comparable with those of the arthropod eye, in 

 precisely the same way and to the same extent as the nervous matter 

 of the brain of the vertebrate is comparable with the brain of the 

 arthropod. In both cases the nervous matter is, in structure, position, 

 and function, absolutely homologous ; in both cases there is found in 

 the vertebrate something extra which is not found in the invertebrate 

 — viz. a hollow tube, the walls of which, in the case of the brain, are 

 utilized as supporting tissues for the nerve structures. The explana- 

 tion of this difference in the case of the brain is the fundamental 

 idea of my whole theory, namely, that the hollow tube is in reality 

 the cephalic stomach of the invertebrate, around which the nervous 

 brain matter was originally grouped in precisely the same manner as 

 in the invertebrate. What, then, are the optic diverticula ? 



