44 HISTOLOGY 



toward the epidermis sinks in upon the deeper layer, transforming the 

 vesicle into the optic cup. At first the cup is not complete, being deficient 

 on its lower side (Fig. 450). The arteria centralis retina is seen passing 

 through this indentation, which begins on the lower surface of the stalk 

 and extends to the free margin of the cup; the cleft is sometimes called 

 the "chorioid fissure." Distal to the point of entrance of the artery into 

 the optic cup, the edges of the fissure fuse; the artery then appears to 

 perforate the base of the cup, and it retains this relation in the adult. 

 The artery is shown in section in Fig. 451, D. 



In a remarkable series of experiments upon tadpoles, Warren Lewis has shown 

 that "the lens is dependent for its origin on the contact influence or stimulus of the 

 optic vesicle." If the optic vesicle is removed, the epithelium in the region of the 

 normal lens does not become thickened or invaginated; but if an optic vesicle is 

 transplanted by detaching it from its stalk and pushing it caudally through the 

 mesenchyma, it will cause the formation of a lens from any portion of the epidermal 

 epithelium which happens to be above it. Moreover, if an area of skin from the ab- 

 domen of a frog of one species is grafted over the optic vesicle of another species, a 

 lens may be produced from the grafted epithelium. Thus there is no predetermined 

 area for lens formation, and its development depends upon the presence of the vesicle 

 beneath (Amer. Journ. Anat., 1904, vol. 3, pp. 505-536, 1907, vol. 7, pp. 145-169). 



The two layers of the optic cup, the inner of 

 which is toward the lens, are normally in contact 

 with one another, although in sections they have 

 often become more or less separated. They consti- 

 tute the retina, which includes a thin outer pig- 

 mented layer, and a thick inner visual layer; the 

 FIG. 450. OPTIC CUP AND latter is composed of several strata of nerve cells and 



STALK OF A HUMAN 



avfter Y Konmann ) ""' n t> ers - The stimulus of light is received by tapering 

 projections extending from the outer surface of the 

 visual layer toward the pigmented layer; to reach them the rays of 

 light must traverse the strata of the visual layer. In explanation of 

 the fact that the sensory processes are turned away from the light, it 

 may be said that the outer surface of the skin ordinarily receives stimuli, 

 and that through the infolding which makes the medullary tube and the 

 outpocketing which makes the optic vesicle, the sensory surface of the 

 retina is continuous with the outer surface of the skin. Since in mammals 

 the optic vesicles begin to form before the related portion of the medullary 

 groove has closed, they first appear as depressions in a thickened epidermal 

 ectoderm. 



Nerve fibers grow from the inner surface of the visual layer toward 

 the central artery and vein of the retina, around which they pass out of 

 the optic cup (Fig. 451, D). They grow beneath and among the cells of 

 the optic stalk to the brain, which they enter. These fibers, which con- 

 stitute the optic nerve, cause the obliteration of the optic stalk. It is 



