494 THE VITREOUS HUMOUR. 



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 cuticular 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 appear 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 

 character of the Ammoccete eye, evidence derived from its structure must 

 be accepted with caution. 



The vitreous humour. The vitreous humour is derived 

 (except in Cyclostomata) from a vascular ingrowth, which differs 

 considerably in different types, through the choroid slit. Its 

 real nature is very much disputed. According to Kessler's view, 

 it is of the nature of a fluid transudation, but the occasional 

 presence in it of ordinary embryonic mesoblast cells, in addition 

 to more numerous blood-corpuscles, gives it a claim to be regarded 

 as intercellular substance. The number of cells in it is however 

 at best extremely small and in many cases there is no trace of 

 them. In Mammals there appear to be some mesoblast cells in- 

 vaginated with the lens, which are not improbably employed in 

 the formation of the vessels of the so-called membrana capsulo- 

 pupillaris. In the Ammoccete the vitreous humour originates 

 from a distinct mesoblastic ingrowth, though the cells which give 

 rise to it subsequently disappear. 



