DEVELOPMENT OF THE SENSE ORGANS 



377 



the lens pit are enclosed in the vesicle and have degenerated in 12.5 mm. embryos 

 (Fig- 35 8 )- At this stage the lens fibers of the proximal wall are longer and this 

 layer will soon obliterate the cavity of the vesicle, as in embryos of 15 to 17 mm. 

 (Fig- 359)- The cells of the distal layer remain of a low columnar type and con- 

 stitute 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 degenera- 

 tion beginning centrally. The fplthelmj, layer 

 structureless capsule of the lens 

 is probably derived from the 

 lens cells. Proximal and distal 

 lens sutures are formed when 

 the longer peripheral fibers over- 

 lap the ends of the shorter cen- 

 tral fibers. These are later trans- 

 formed into "lens-stars" (Fig. 

 360). The lens, at first some- 

 what triangular in cross section, 

 becomes nearly spherical at 

 three months (Fig. 360). 



The origin of the vitreous 

 body has been in doubt, one 

 view deriving it from the mesen- 

 chyma which enters the optic 



cup through the chorioid fissures and about the edge of lens, another view hold- 

 ing that it arises from cytoplasmic processes of cells in the retinal layer. 



It is certain that the vitreous tissue is formed before mesenchyma is present in the cavity 

 of the optic cup. Szily regards this primitive vitreous body as a derivative of both retinal and 

 lens cells, it forming a non-cellular network of cytoplasmic processes which are continuous 

 with the cells of the lens and retina. With the ingrowth of the central artery of the retina, 

 from which the artery of the lens passes to and branches on the proximal surface of the lens, 

 a certain amount of mesenchymal tissue invades the optic cup and this tissue probably con- 

 tributes to the development of the vitreous body (Fig. 358). 



The vitreous body may therefore be regarded as a derivative both of the ectoderm and 

 of the mesoderm. 



Lens fibers 



^Ectoderm 



Fig. 359. — Section through the lens and corneal ecto- 

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



The mesenchyma accompanying the vessels to the proximal surface of the 



