V.] THE OPTIC NERVE. 107 



glionic layer are distinguishable. Very early the substance of 

 certain of the cells takes on the appearance of fibres, arranged 

 vertically, i. e. radiating from the inner or anterior surface of 

 the retina to the membrana limitans externa. These are 

 the rudiments of the fibres of Miiller. 



Thus of the cells of the inner wall of the cup, some 

 becoine ganglionic cells, and others the fibres of Miiller, 

 while the nuclei of yet others remain as the inner and outer 

 granules. The rods and cones are outgrowths of the cells to 

 which the outer granules belong. All pa.rts of the retina, 

 in fact, whether simply connective, or really nervous in nature, 

 seem to be derived from epiblastic cells. 



The changes described above are confined to that portion 

 of the retina which lies behind the ora serrata. In front of 

 this both walls of the cup coalesce as we have said into a 

 cellular layer in which a deposit of pigment takes place. 



The optic nerve. Histological changes are first observable 

 in the optic stalk at about the time when its cavity loses all 

 connection with the cerebral hemisphere and opens ex- 

 clusively into the third ventricle.. It is then that fibres first 

 make their appearance in its walls, nuclei being still abun- 

 dantly present. The stalk though much elongated is still 

 hollow and its cavity is circular in section. According to 

 Lieberkiihn at no time does it (in the bird) undergo any 

 involution tending to obliterate its cavity. 



Soon after the deposition of pigment in the outer wall of 

 the optic cup, while the optic stalks are as yet still hollow, 

 the rudiments of the optic chiasma appear. The fibres of 

 the one stalk grow over into the attachment of the other. 

 About the same time the fibres at the neck of the optic cup 

 grow forward and become connected with the retina, over whose 

 internal surface they spread. The entrance of the optic 

 nerve into the eyeball is closely connected with that of the 

 pecten, its fibres passing in at the lower end of that body, 

 coursing along its sides to its upper end and radiating from 

 it as from a centre to all parts of the retina. 



Before the exclusion of the chick the optic nerve becomes 

 solid by the gradual filling up of its central cavity. 



The lens when first formed is somewhat elliptical in 

 section with a small central cavity of a similar shape, 

 the front and hind walls being of nearly equal thickness, 



