DEVELOPMENT OE THE SENSE ORGANS 



379 



gives rise to the sclerotic layer, to the chorioid layer, and to the anterior layers 

 of the ciliary body and iris. 



Both the inner and outer layers of the optic cup are continued into the optic 

 stalk, as seen in Fig. 376. This is due to the trough-like invagination of the 

 ventral wall of the optic stalk j the chorioid fissure, when the optic vesicle is trans- 

 formed into the optic cup (Fig. 375). Into the chorioid fissure grows the central 

 artery of the retina, carrying with it into the posterior cavity of the eye a small 

 amount of mesenchyme (Fig. 377). Branches from this vessel extend to the 

 posterior surface of the lens and supply it with nutriment for its growth. At a 

 later stage the chorioid fissure Epithelial layer 



closes, so that the distal rim of 

 the optic cup forms a complete 

 circle. 



Capsule 



Vascular 

 membrane 



Lens fibers 



If the chorioid fissure fails to 

 close, the optic cup remains open at 

 one point and this results in the de- 

 fective development of the iris, ciliary 

 body, and chorioid layer. Such a 

 defect is known as coloboma. 



The old view that the develop- 

 ment of the lens vesicle causes the 

 formation of the optic cup by push- 

 ing in its distal wall has been dis- 

 proved by W. H. Lewis, for if an 

 anlage of the optic vesicle from an 

 amphibian embryo be transplanted 

 to some other part of the embryo, 

 it can develop into an optic cup in 

 the absence of a lens. Furthermore, 

 it is the contact of optic vesicle with 

 ectoderm that furnishes the stimulus for lens formation, both normally and after trans- 

 plantation to foreign regions, e. g., abdomen. 



The lens vesicle, and its early development from the ectoderm, have been de- 

 scribed. Its proximal wall is much thickened in 10 mm. embryos, and these 

 cells form the lens fibers (Fig. 376) which will soon obhterate the cavity of the 

 vesicle, as in embryos of 15 to 17 mm. (Fig. 378). The cells of the distal layer 

 remain of a low columnar type and constitute the epithelial layer of the lens. 

 When the lens fibers attain a length of 0.18 mm. they cease forming new fibers by 

 cell division. New fibers thereafter arise from the cells of the epithelial layer at 

 its line of union with the lens fibers. The nuclei are arranged in a layer convex 

 toward the outer surface of the eye and later degenerate, the degeneration begin- 



Ectoderm 

 Fig. 378. — Section through the lens and corneal ecto- 

 derm of a 16 mxa. pig embryo. X 140. 



