VI.] THE LENS. 149 



ating from it as from a centre to all parts of the 

 retina. 



The lens. This 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, each consisting of a single layer of 

 elongated columnar cells. 



In the subsequent growth of the lens, the develop- 

 ! ment of the hind wall is of a precisely opposite cha- 

 racter 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 con- 

 trary becomes thinner and thinner and its cells more 

 and more flattened and pavement-like. 



These modes of growth continue until at the end of 

 the fourth day, as shewn in Fig. 51, the convex hind 

 wall I comes into absolute contact with the front wall 

 el and the cavity is thus 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, some- 

 what thickened at either side where it becomes continu- 

 ous 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 this it becomes the epithelium. 



The subsequent changes undergone consist chiefly in 

 the continued elongation and multiplication of the lens- 

 fibres, with the partial disappearance of their nuclei. 



