576 TEXT-BOOK OF EMBRYOLOGY. 



cells of the mature vitreous after the blood vessels have degenerated is not 

 known. 



As already noted, the vitreous is at first crossed by the hyaloid artery which 

 supplies the developing lens (p. 569). As lens formation becomes less active 

 the artery becomes less important and by the end of the third month begins to 

 atrophy. At birth nothing remains of it, but in its former course the vitreous 

 is somewhat more fluid than elsewhere and this is known as the hyaloid canal 

 (canal of Cloquet). 



The Optic Nerve. Referring to the description of the optic evagination it 

 will be recalled that the optic vesicle maintains its connection with the brain by 

 means of the optic stalk (p. 564). The latter is hollow and connects the cavity 

 of the optic vesicle with the cavity of the brain. When the invagination of the 

 optic vesicle to form the optic cup occurs (p. 566, Fig. 502), the invagination is 

 carried along the posterior surface of the optic stalk toward the brain, and just 

 as the invagination of the optic vesicle results in the obliteration of the cavity 

 of the vesicle, so the invagination of the optic stalk results in an oblitera- 

 tion of its lumen. In Mammals the invagination of the optic stalk extends only 

 part way to the brain, to the point where the artery enters. The chorioidal 

 fissure closes about the seventh week. 



The optic stalk consists of supportive elements only, and serves as a track 

 along which nerve fibers extend to connect the retina and brain. Nerve fibers 

 appear in the optic stalk about the fifth week. They appear first around the 

 periphery and apparently crowd the neuroglia nuclei toward the center, so that 

 the stalk at this stage may be said to consist of a mantle layer and a marginal 

 layer, apparently analogous to these layers in the retina and brain. The nerve 

 fibers gradually invade the entire stalk so that by the end of the third month the 

 stalk has become transformed into the optic nerve among the fibers of which the 

 original supportive elements of the stalk are still represented by neuroglia cells. 



Much difference of opinion has existed in regard to the origin of the optic 

 nerve fibers, whether they are processes of retinal cells which end in the brain 

 or processes of brain cells which end in the retina. It is now quite generally 

 accepted that most of the fibers of the optic nerve are the axones of neurones the 

 cell bodies of which are situated in the ganglion cell layer of the retina. These 

 axones pass centrally into the layer of nerve fibers, which they form, and con- 

 verge toward the optic nerve. Through the latter they pass to their terminations 

 in the external geniculate bodies, optic thalami and anterior corpora quadri- 

 gemina. According to Cajal and others, some centrifugal fibers are present in 

 the optic nerve. These are processes of cells situated in the above-mentioned 

 nuclei, and terminate in the retina. They are fewer in number and of later 

 development than the centripetal fibers. 



As the mesodermic anlagen of the chorioid and sclera are present before 



