LENS 



139 



The thin anterior layer remains throughout life as a simple layer of cubical 

 cells, and forms the so-called lens-epithelium ; but the cells of the posterior layer 

 grow forwards into the cavity of the lens-vesicle as the lens-fibres : the central 

 fibres are the longest and straight (fig. 182), while the rest are slightly curved 

 with their concavity towards the equator. The fibres become gradually shorter 

 towards the circumference, where they pass through gradually shortening 





FIG. 182. SECTION OF THE DEVELOPING EYE OF A BABBIT-EMBBYO OF THE THIRTEENTH DAY. 



(T. H. Bryce.) 



The section passes through the optic stalk, and cuts the groove in which the central artery of the 

 retina passes into the interior of the optic cup. The cavity of the lens-vesicle is not yet obliterated ; 

 its anterior wall is formed of a layer of cubical cells its posterior wall, greatly thickened, is becoming 

 converted into the lens-fibres. The cavity of the optic cup is almost filled by the lens. On the surface 

 of the retinal layer of the cup is seen a protoplasmic nuclear-free zone which is the primitive vitreous. 

 It has shrunk away from the lens, and thereby shows very clearly the vascular layer of mesenchyme on 

 the posterior aspect of that body. The vessels are seen entering the optic cup through the choroidal 

 fissure of the optic stalk, and also through the space between the lens and the mouth of the cup. 



columnar cells (transitional zone) into continuity with the anterior epithelium. 

 By the growth of these fibres the cavity of the lens-vesicle becomes obliterated. 



In this manner the central part of the lens is developed, and it consists in the 

 main of fibres which pass in an antero-posterior direction. The remainder of 

 the lens is formed of fibres which are so disposed as to curve round its margin 

 and over the ends of the first formed fibres ; they are, moreover, deposited in 



