DEVELOPMENT OF THE SENSE ORGANS 



379 



Epithelial layer 



Mesenchymal tissue surrounds the optic cup and is beginning to make 

 its way between the lens vesicle and the ectoderm. Here, the anterior 

 chamber of the eye develops' later as a cleft in'the mesoderm. The dis- 

 tal mesenchymal tissue .(next the ectoderm) forms the substqntia propria 

 of the cornea and its posterior epithelium, while the proximal mesen- 

 chyma (next the lens) differentiates into tbe, vascular capsule of the lens. 

 The mesenchyme surrounding the optic cup is continuous with that which 

 forms the cornea; later it 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 state, as seen in Fig. 

 376. This is due to the trough-' 

 like invagination of the ventral 

 wall of the" optic stalk, the 

 chorioid fissure, when the optic 

 vesicle is transformed 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 ex- 

 tend to the posterior surface 

 of the lens and supply it with 

 nutriment for its growth. At a 

 later stage the chorioid fissure 

 closes, so that the distal rim of 

 the optic cup forms a complete 

 circle. 



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

 been described. Its proximal wall is much thickened in 10 mm. embryos 

 (Fig. 376), and these cells form the lens fibers which will soon obliterate 

 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 

 o.i 8 mm. they cease forming new fibers by cell division. New fibers 

 thereafter arise from the cells of the epithelial layer at its equatorial 

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

 convex toward the outer surface of the eye, but they later degenerate, 

 the degeneration beginning centrally. Lens sutures are formed on the 

 proximal and distal faces of the lens when the longer, newly formed, 



Lens fibers 



Ectoderm 



FIG. 378. Section through the lens and corneal 

 ectoderm of a 16 mm. pig embryo. Xi4O. 



