400 THE LEXS. 



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 Avhich tiie 

 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 I is in absolute contact with the front wall el, and the 

 cavity thus becomes entirely obliterated. The cells of the hind wall 

 have by this time become veritable fibres, which, when seen in section, 

 appear to be arranged nearly parallel to the optic axis, their nuclei 

 nl being seen in a row along their middle. The front wall, somewhat 

 thickened at either side where it becomes continuous with the hind 

 wall, is now a single layer of flattened cells separating the hind wall 

 of the lens, or as we may now say the lens itself, from the front limb 

 of the lens-capsule ; of the latter it becomes the epithelium. 



The subsequent changes undergone consist chiefly in the con- 

 tinued elongation and multiplication of the lens-fibres, with the 

 partial disappearance of their nuclei. 



During their multiplication they become arranged in the manner 

 characteristic of the adult lens of the various forms. The lens-capsule, 

 as was originally stated by Kolliker, appears to be formed as a cuticu- 

 lar membrane deposited by the epithelial cells of the lens. 



The views of Lieberkiihn, Arnold, Lowe and others, according to 

 which the lens-capsule is a mesoblastic structure, do not ai)pear to be well 

 founded. The contrary view, held by Kolliker, Kessler, etc., is supported 

 mainly by the fact that at the time when the lens-capsule first appears 

 there are no mesoblast cells to give rise to it. It should however be stated 

 that W. Miiller has actually found cellular elements in what he believes to 

 be the lens-capsule of the Ammoccete lens. Considering the degraded 



