THE EMBRYOLOGY OF THE EYE. 53 



of the coloboma of the iris, but the conse- 

 quence. 



9. Retina and pigment-epithelium, 

 a. Retina. When the secondary vesicle is 

 fully formed its inner layer from which the 

 retina develops, is bordered on either surface 

 by a more or less distinct membrane, which 

 membranes become respectively the limitans 

 externa and the limitans interna of the retina 



(Fig- 4). 



The cells of the stratum next the limitans 

 interna are columnar and the nucleus lies some 

 little distance from the membrane (Fig. 4). A 

 little later, these columnar cells become spindle- 

 shaped and small spaces appear between their 

 internal prolongations. When the limitans 

 interna is stripped off, these prolongations are 

 seen to form a fine network (Fig. 7). A 

 similar network is found on the margin of the 

 wall of the cerebral vesicles. At the next 

 stage we find nerve-fibres extending continu- 

 ously from the cerebral vesicle through the 

 optic nerve to the retina (Fig. 8). The inter- 



