ORGANS OF VISION OF THE VERTEBRATA. 493 



cavities gradually become obliterated by a thickening of the 

 walls, the obliteration proceeding from the retinal end inwards 

 towards the brain. While the proximal ends of the optic stalks 

 are still hollow the rudiments of the optic chiasma are formed 

 from fibres at the roots of the stalks, the fibres of the one stalk 

 growing over into the attachment of the other. The decussation 

 of the fibres would appear to be complete. The fibres arise in 

 the remainder of the nerves somewhat later. At first the optic 

 nerve is equally continuous with both walls of the optic cup ; as 

 must of necessity be the case, since the interval which primarily 

 exists between the two walls is continuous with the cavity of the 

 stalk. When the cavity within the optic nerve vanishes, and 

 the fibres of the optic nerve appear, all connection is ruptured 

 between the outer wall of the optic cup and the optic nerve, and 

 the optic nerve simply perforates the outer wall, and becomes 

 continuous with the inner one. 



There does not appear to me any ground for doubting (as has 

 been done by His and Kolliker) that the fibres of the optic nerve 

 are derived from a differentiation of the epithelial cells of which 

 the nerve is at first formed. 



Choroid Fissure. With reference to the choroid fissure we 

 may state that its behaviour varies somewhat in the different 

 types. It becomes for the greater part of its extent closed, 

 though its proximal end is always perforated by the optic nerve, 

 and in many forms by a mesoblastic process also. 



The lens when first formed is an oval vesicle with a small 

 central cavity, the front and hind walls being of nearly equal 

 thickness, and each consisting of a single layer of elongated 

 columnar cells. In the subsequent stages the mode of growth 

 of the hind wall is of precisely an opposite character to that of 

 the front wall. The hind wall becomes much thicker, and tends 

 to obliterate the central cavity by becoming convex on its front 

 surface. At the same time its cells, still remaining as a single 

 layer, become elongated and fibre-like. The front wall on the 

 contrary becomes thinner and thinner and its cells flattened. 



These modes of growth continue until, as shewn in fig. 289, 

 the hind wall / is in absolute contact with the front wall <?/, and 

 the cavity thus becomes entirely obliterated. The cells of the 

 hind wall have by this time become veritable fibres, which, when 



