308 PROCEEDINGS OF THE ACADEMY OF [1884. 



eye, being separated by a layer of fibres, which is the much- 

 shortened and flattened primitive optic nerve. The pedicle 

 connecting this advanced part of the brain, which may be looked 

 upon as a ganglion, we will now call the " secondar}^ optic nerve," 

 the optic nerve of the eyes of the adult Vertehrala. A similar 

 state of affairs as this is found to-day in the eyes of the Cephalo- 

 poda dibranchiata. This ganglion soon becomes the most impor- 

 tant part of the eye, and receives the light-waves upon its 

 exterior wall, the primitive eye becoming transparent, and later 

 forming the lens. This " ganglion opticum,'''' as it may be provi- 

 sionally called, gradually proceeds downwards about the primi- 

 tive eye, joining below. As development and importance 

 advance, we find the hollowing out of the ganglion opticum, 

 this structure later is filled with the corpus vitreum, which is 

 included, as was shown in the development of the eyes of the 

 Vertebrata. Thus, I hold, if this hypothesis be a true one, that 

 (1) the lens of the eyes of the Vertebrata is homologous with a 

 primitive invaginated eye, such as we find to-day in the gastro- 

 poda ; that (2) the layer of optic fibres of the retina is homolo- 

 gous with the primitive optic nerve. As the retina below has 

 become the sensory part of the e3'e, the rays of light must 

 necessarily pass through it, to reach a point where nerve-energy 

 is developed. The nervus opticus of the eyes of the Vertebrata 

 is, therefore, according to this view, really a secondary optic 

 nerve. 



We find in the vertebrata, and much more frequently in the 

 invertebrata, blind animals, the near relatives of which have well- 

 developed organs of sight. This blindness is due to the peculiar 

 environments of the animal, such as cave life, where light is 

 excluded ; parasitism, etc., etc. 



The Proteus of the Adelsberg grotto is an animal that is practi- 

 cally devoid of pigment. The eye of this practically blind animal 

 is remarkable, inasmuch, that no lens is developed in the adult 

 state. Our literature is unfortunately deficient in the embryology 

 of this interesting form, so it is at present a matter of impossi- 

 bility to state whether there ever exist a lens in the earl}'^ 

 development of the eye. The primitive optic vesicle has the 

 form of that of the embr3^os of those vertebrates which have 

 well-developed eyes in the adult state ; the retina is a thick, 

 many-celled layer, l^'ing on the stratum pigmcntum, which contains 



