THE ORGANS OF THE OUTER GERM-LAYER. 485 



Originally the optic nerve is a tube with a small lumen, which 

 unites the cavity of the optic vesicle with the third ventricle 

 (fig. 264 A). It is gradually converted into a solid cord. In the 

 case of most Vertebrates this is produced simply by a thickening of 

 the walls of the stalk, due to cell-proliferation, until the cavity is 

 obliterated. In Mammals only the larger portion, that which adjoins 

 the brain, is metamorphosed in this manner ; the smaller part, that 

 which is united with the optic vesicle, is, on the contrary, infolded by 

 the prolongation of the choroid fissure backward for some distance, 

 whereby the ventral wall is pressed in against the dorsal. Con- 

 sequently the optic nerve here assumes the form of a groove, in 

 which is imbedded a connective-tissue cord with a blood-vessel that 

 becomes the arteria centralis retinae. By the growing together of 

 the edges of the groove, the cord afterwards becomes completely 

 enclosed. 



For a time the optic nerve consists exclusively of spindle-shaped, 

 radially arranged cells in layers, and resembles in its finer structure 

 the wall of the brain and the optic vesicle. Different views are held 

 concerning its further metamorphoses, and especially concerning the 

 origin of nerve-fibres in it. Differences similar to those concerning 

 the origin of the peripheral nerve-fibres are maintained. Upon this 

 point three theories have been brought forward. 



According to the older view, which LIEBERKUHN shares, the optic 

 fibres are developed in loco by the elongation of the spindle-shaped 

 cells. According to His, KOLLIKER, and W. MULLER, on the con- 

 trary, the wall of the optic vesicle furnishes the sustentative tissue 

 only, whereas the nerve-fibres grow into it from outside, either from 

 the brain toward the retina (His, KOLLIKER), or in the reverse direction 

 (MULLER). The stalk of the optic vesicle would constitute, according 

 to this view, only a guiding structure as it were would predeter- 

 mine the way for its growth. When the ingrowth has taken place, 

 the sustentative cells are, as KOLLIKER describes them, arranged 

 radially and so united with one another that they constitute a 

 delicate framework with longitudinally elongated spaces. In the 

 latter are lodged the small bundles of very fine non-nuclear nerve- 

 fibres and numerous cells, arranged in longitudinal rows, which 

 likewise belong to the epithelial sustentative tissue and help to 

 complete the trestie-work. 



The embryonic optic nerve is enveloped in a connective-tissue 

 sheath, which is separated, as in the case of the brain and secondary 

 optic cup, into an inner, soft, vascular and an outer compact 



