172 THE GERM LAYER DERIVATIVES 



opening of the optic cup. This is brought about under the inductive 

 influences emanating from the dorsal rim of the optic cup by the 4 

 mm. stage, some time before hatching. The lens originates as a placode 

 or thickening in the inner or neural layer of the head ectoderm. This 

 placode invaginates to form a (lens) vesicle by the 5 mm. stage. This 

 vesicle pinches off from the head ectoderm and comes to lie within the 

 ring of the optic cup and is supported by a suspensory ligament by 

 the 6 mm. stage. The outer wall of the lens vesicle remains as cuboidal 

 epithelial cells and the inner wall cells become elongated as lens fibers. 

 The cavity is ultimately obliterated. The head ectoderm then closes 

 over the lens to form a new covering which, in conjunction with the 

 head mesenchyme, forms the double-layered cornea. This cornea is 

 therefore derived from ectoderm and mesoderm and becomes a trans- 

 parent covering of the lens by the 6 mm. stage. 



After hatching, at about the 6 mm. stage, the outermost wall of the 

 optic cup is seen as a pigmented layer which comes into contact with 

 the thick inner retinal wall which thereupon begins to give rise to the 

 rods and cones from its outer margin. These visually sensitive elements, 

 the rods and cones, are fully developed by the 1 1 mm. stage. They 

 point away from the light source, their posteriorly directed axons 

 covering the exposed face of the retina. The inner margin of the retina, 

 i.e., that toward the optic cup, is made up of neuroblasts and their 

 jSbers (axons) which pass over the surface of the retina to leave the 

 optic cup together by way of the choroid fissure. They then pass along 

 the walls of the optic stalk, which acts as a guiding path to the 

 fibers, to reach the diencephalon of the brain as the second cranial or 

 the optic nerves. There is a junction and crossing of the paired optic 

 nerve fibers in the optic chiasma which is seen in the floor of the dien- 

 cephalon. The choroid fissure will eventually close around the blood 

 vessels and nerves which supply the optic cup. These latter nerves 

 have no visual function. The ventral margins of the optic cup, where 

 the choroid fissure originates, eventually fuse to form the choroid knot 

 and it is from this cluster of cells that the cells of the iris arise. Pigment 

 very soon disappears from the outer superficial layer of the original 

 optic cup. 



The large cavity of the eye between the lens and the retina, des- 

 ignated as the optic cup, becomes filled with a viscous fluid known as 

 the vitreous humor. This is derived from the cells of the retinal wall 

 and lens and is therefore of ectodermal origin. Head mesenchyme 



