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 the 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 Miiller’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. Wesee, 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 ? 
