250 DR WALTER H. GASKELL. 



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 

 regard to the existence of the tube, Gotte's observation that 

 the true retina has been formed from the Sinnes-platte (optic 

 plate) 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 the description of the 

 development of the compound retina of the Arthropod eye, as 

 has been already stated. 



The Meaning of the Optie Divcrtieula. 



The origin of the retina from an optic epidermal plate in 

 vertebrates, 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. 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 homo- 

 logous ; 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 utilised as 

 supporting tissues for the nerve structures. The explanation 

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

 idea of my whole theory, viz., 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 ? 



"The formation of the eye," as taught by Balfour,^ "com- 

 mences with the appearance of a pair of hollow outgrowths 

 from the anterior cerebral vesicle. These outgrowths, known 

 as the optic vesicles, at first open freely into the cavity of the 

 anterior cerebral vesicle. From this they soon, however, 



' Op. c!t., \\ 398. 



