THE SENSE ORGANS 



589 



Where the optic vesicle, as it expands, comes into contact with the 

 ectoderm, a local thickening of the ectoderm forms as the anlage of the 

 lens. This placode thickens, invaginates, and sinks below the surface to 

 form a small hollow vesicle, which eventually loses connexion with the 

 skin. The lens vesicle is finally converted into a solid lens by the thicken- 

 ing of its medial wall, the epithelial cells of which become elongated into 

 fibers and arranged in layers like the coats of an onion. 



Fig. 486. — Sections of successive stages in the development of the lens of the eye 

 from the first thickening of the ectoderm (ec) to the complete separation of the lens, /. 

 (From Kingsley's "Comparative Anatomy of Vertebrates.") 



While the lens is undergoing these changes, it becomes enclosed by 

 the optic cup, formed by invagination of the optic vesicle. Possibly the 

 ingrowth and enlargement of the lens is a factor in this invagination. The 

 optic vesicle is thus converted into a two-layered optic cup, attached to 

 the brain by the optic stalk. The cup is, however, incomplete, for a fissure 



CONJUNCTIVA- 

 CORNEA- 

 AQUEOUS HUMOR- 

 IRIS 



WITREOUS BODV 



Fig. 487. — Diagram illustrating the growth of optic nerve fibers from the retina 

 along the optic stalk into the brain. Some of the fibers cross below the brain to form 

 the optic chiasma, while the remaining pass into the thalamus of the same side. 



divides it along its lower side, and extends as a groove along the lower side 

 of the optic stalk. 



Of the two layers of the optic cup, the outer persists as the single- 

 layered pigment epithelium, and the inner thickens to form the three 

 layers of neurons of the definitive retina. Neurites from the inner 

 ganglion cell layer grow along the optic stalk and gradually fill its groove. 

 Half of these fibers cross to the opposite side of the brain to form the 

 chiasma of the optic nerve. The cells of the optic stalk are converted 

 into neuroglia cells. 



