160 BIOLOGY: GENERAL AND MEDICAL 



fibres with which they are connected converging to 

 form an optic nerve. Eyes of this kind persist among 

 the arthropods, though more highly specialized eyes 

 are also present. 



From such eyes it is a simple step to the camera-eye, in 

 which the nervous elements surround a spherical space 

 into which the light comes through a minute opening, 

 the homologue of the pupil, causing an inverted image 

 to fall upon the more numerous nerve elements and 



Via. 60. 



FIG. 60. : Diagram illustrating the early development of the vertebrate eye, 

 (Galloway.) 



b.v. The brain vesicle formed by the invagination of the ectoderm, eel.; 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. 61. Diagram illustrating a later developmental stage of the vertebrate 

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

 mes, mesodermal tissue. (Galloway.) 



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 

 up of hundreds of units consisting of an outer ectodermal 

 transparent cuticle or cornea, beneath which are pig- 



