58 THE EMBRYOLOGY OF THE EYE. 



anteriorly, and here it is always denser and 

 more resistant to the action of bleachincr 

 agents even in adult life. The margins of the 

 inferior cleft are not pigmented until a con- 

 siderable time after the cleft is obliterated. 

 The pigment gradually fills the entire cell, ex- 

 cepting the nucleus and the outer margin, 

 which always remains free. 



In the chick the deposition of pigment 

 occurs somewhat later and begins in the outer 

 portion of the cell, the pigment extending to 

 the outer margin. Later, when the anterior 

 cells become columnar, the pigment Is most 

 dense in the inner portion of the cell. 



lo. Optic nerve. — Just before the lens is 

 fully formed, and while the eyelids are only 

 indicated by two small folds (Fig. 4), w^e find 

 the inferior cleft of the secondary vesicle 

 closed but still showing evidences of its recent 

 closure. In Fig. 16, A is an equatorial sec- 

 tion through the inferior wall of the vesicle, 

 showing the inner layer folded where the cleft 

 has closed ; B is a section nearer the optic 



