DEVELOPMENT OF THE EYE. 353 



EYE. 



DEVELOPMENT AND GENERAL ANATOMY. 



The eyes first appear as a pair of optic vesicles, which are lateral out- 

 pocketings of the fore-brain. They are shown in the model, Fig. 390, A 

 (p. 325) and in section in Fig. 409, A. The vesicles are connected with the 

 brain by the optic stalks, which become relatively slender as the vesicles 

 enlarge. The epidermal ectoderm immediately overlying the vesicles, 

 thickens and becomes invaginated (Fig. 409, B and C). The invaginated 

 portion then becomes detached in the form of a vesicle, the inner wall of 

 which is distinctly thicker than the outer; this "lentic vesicle" becomes 

 the lens of the eye. Meanwhile, as seen in B and C, that layer of the optic 

 vesicle which is toward the surface is pressed in, transforming the vesicle 

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

 lower side (Fig. 408). The arteria centralis retinae is 

 seen passing through the 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 



FIG. 408. OPTIC CUP 



of the cup, and it retains this relation in the adult. AND STALK OP A 



HUMAN EMBRYO 



The artery is shown in section in Fig. 409, D. S?,, 6 - 9 M ^- (After 



* K.ollmann.; 



The two layers of the optic cup, the inner of 



which is toward the lens, are normally in contact with one another, al- 

 though in sections they are often more or less separated. They constitute 

 the retina, which includes a thin outer pigmented layer, and a thick inner 

 visual layer composed of several strata of nerve cells and fibers. 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 is stated 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 sur- 

 face of the retina is seen to be continuous with the outer surface of the 

 skin. Since in mammals the optic vesicles begin to form before the rela- 

 ted portion of the medullary groove has closed, they appear as depressions 

 in a thickened epidermal ectoderm. 



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

 23 



