DIFFERENTIATION OF RETINA; LENS 109 



simply by examining regions which are successively farther from the 

 posterior pole and nearer to the ora terminalis, or rim of the cup. An- 

 other consequence is that the region of the ora is to some extent perma- 

 nently juvenile. If the retina is destroyed and subsequently regenerates 

 (as it will do in amphibians, though not in any other vertebrates) the 

 new retina grows from the ora terminalis, creeping backward until the 

 fundus is filled; and a new optic nerve develops pari passu with the 

 regeneration. A perennial mystery, however, is the fact that the retina 

 continues to increase greatly in extent after all cell-division has appar- 

 ently ceased in it — as is the case, for instance, in an amphibian which is 

 on the verge of metamorphosis, though the eye is then nowhere nearly 

 adult in size. It is possible that sensory-retinal elements continue to 

 differentiate from the ciliary epithelial cells (v.i.) at the ora, and the 

 application of the colchicine technique may solve this problem. 



The Lens — All this time, the lens has been developing. Commencing 

 as a local thickening in the surface ectoderm, the 'lens placode' evoked 

 by some chemical emanation from the contiguous optic vesicle, it has 

 invaginated and pinched free of the surface ectoderm, which heals over 

 it without trace. 



Thus is formed the lens vesicle, lying in the mouth of the optic cup 

 (Fig. 38c'). Its posterior or inner wall rapidly thickens, each of the 

 cuboidal cells becoming columnar and continuing to elongate until it 

 is a slender fiber. The growth in length of these cells being in a forward 

 direction, they encroach upon the cavity of the lens vesicle and oblit- 

 erate it, forming a solid mass whose anterior surface is still covered over 

 by the unmodified cuboidal cells of the original anterior wall of the 

 vesicle (Fig. 40). This cuboidal layer is the lens epithelium, and at the 

 equator of the lens it forever remains continuous with the greatly thick- 

 ened posterior wall. In this region of transition, epithelial cells now 

 commence to elongate and to rotate their axes of polarity until they are 

 no longer radially oriented with respect to the center of the lens, but 

 circumferentially disposed (Fig. 41a). The two ends of these elongating 

 equatorial cells disconnect from the ends of their neighbors in the epi- 

 thelium and grow apace, one end sliding forward under the epithelium 

 and the other end backward, guided by the confining capsule which has 

 already been secreted by the lens vesicle over its whole outer surface. 



Thus a layer of circumferential lens fibers is laid down, like one of 

 the skins of an onion, over the central mass of original, straight lens 



