DEVELOPMENT OF EYE OF SPARROW 265 



EYEBALL IN GENERAL 



The development of the eyes in all vertebrates follow the same 

 general plan. They first appear as lateral projections from the 

 forebrain, known as the primary optic vesicles (text-fig. opv.). 



Text fig. 1 Dorsal view of the embryo chick at the age of about thirty hours. 

 (See fig. 4,3.) e, auditory pit;/6, fore-brain; hb, hind-brain; jnh, mid-brain; mg, 

 medullary groove; m.s, mesoblastic somites; opv, optic vesicle. 



Text fig. 2 Diagrams showing the development of the lens and the optic cup. 

 (See figs. 44 and 45.) e, ectoderm; /, lens; opv, optic vesicle; pg, posterior (pig- 

 ment), and r, anterior (retinal) walls of the primary optic vesicle. 



Text fig. 3 A, diagram of a vertical section of the eye of the chick at about 

 forty -four hours' incubation in the plane s-s of figures 2, C, and 5. B, diagram 

 of a section made vertical to A along the line s-s. (See fig. 45.) chf, chorioid 

 fissure; I, lens; Ic, lens vesicle; op st, optic stalk; pg, posterior (pigment) wall 

 of the primary optic vesicle; popv, optic vesicle; r, anterior (retinal) wall of 

 optic vesicle. 



These vesicles continue their growth laterally until they come in 

 contact with the ectoderm (text-fig. 2, A), the cells of which at 

 this point become elongated (text-fig. 2, A Z) . This thickening is 

 the beginning of the lens. Figure 43 is a photograph of a cross- 

 section of a chick embryo of about forty hours' incubation. It 

 shows the two optic vesicles {Opv) projecting laterally and 



