THE HIGHER ORGANISMS 161 



so effecting a differentiation of lights and shadows. 

 The next specialization consists in an outer transparent 

 cuticular covering or cornea, the presence of a clear jelly 

 in the space vitreous body and eventually of a lens 

 by which the light rays are refracted and accurately 

 distributed. There are many variations of the ap- 

 paratus, however, for in the arthropods it develops into 

 a congeries of what might be described as visual units, 

 as in the compound eye of the insects which are made 



FIG. 61. 



FIG. 61. Diagram illustrating the early development of the vertebrate eye. 

 (Galloway.) 



b.v, The brain vesicle formed by the invagination of the ectoderm, ect.; mes, 

 mesodermal tissue; os, optic stalk; ov, optic vesicle, a portion of the brain 

 vesicle; I, lens. The right side of the figure shows a slightly later developmental 

 stage than the left. 



FIG. 62. Diagram illustrating a later developmental stage of the vertebrate 

 eye. On, optic nerve; r, retina; v.h, vitreous humor; I, lens; ect, ectodennal tissue; 

 mes, mesodermal tissue. (Galloway.) 



up of hundreds of units consisting of an outer ectodermal 

 transparent cuticle or cornea, beneath which are pig- 

 ment cells with subjacent nervous elements in groups. 

 The images gathered by such eyes may be regarded as a 

 kind of mosaic made up of many small bits. There 

 can be no accommodation and no perspective. From 

 such eyes great bundles of nerve fibres pass to the 

 optic lobes of the brain, so increasing its complexity. 

 Among certain mollusks, such as the cephalopods, the 

 eye forms a beautiful and striking organ superficially 

 ii 



