THE EMBRYOLOGY OF THE EYE. 33 



involution of the neural epiblast to form the 

 cerebro-spinal axis, and again in the closure of 

 the cleft in the ventral wall of the secondary 

 optic vesicle. 



The wall of the lens-sac consists of a uni- 

 form layer of epithelial cells, four or five deep, 

 both in the chick (Fig. 3) and in the rabbit 

 (Fig. 4). The cells of the posterior layer 

 lengthen out into fibres. The beginning of 

 this process is shown in Fig. 4 below. These 

 fibres, developing more rapidly in the centre, 

 push forward until they reach the anterior 

 layer, and thus obliterate the cavity of the 

 sac (Fig. 6). Each fibre has a single nucleus, 

 and is developed from a single cell. In the 

 embryo-pig at the time when the lens is fully 

 formed, the anterior epithelium is four or five 

 cells deep (Fig. 7). The short fibres which 

 develop at the angles of the lens-sac curve 

 sharply outward to reach the anterior epi- 

 thelium, those nearer the posterior pole curve 

 less, and those arising in the polar region 



extend forward in a straight line (Figs. 6 and 

 3 



