138 



THE THIRD DAY. 



[CHAP. 



in the diagram 



much the appearance represented 

 Fig. 49. 



A section of such an eye taken along the line y, 

 perpendicular to the plane of the paper, would give a 

 figure corresponding to that of Fig. 50 A. The lens, 

 the cavity and double walls of the secondary vesicle, and 

 the remains of the primary cavity, would all be repre- 



DlAGRAMMATIC REPRESENTATION OF THE EYE OF THE CHICK 

 OF ABOUT THE THIRD DAY AS SEEN WHEN THE HEAD IS 

 VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT. 



I the lens, I' the cavity of the lens, lying in the hollow of the 



optic cup. 

 r the anterior, u the posterior wall of the optic cup, c the cavity 



of the primary optic vesicle, now nearly obliterated. By 



inadvertence u has been drawn thicker than r, it should 



have been thinner throughout. 

 s the stalk of the optic cup with s its cavity, at a lower level 



than the cup itself and therefore out of focus ; the dotted 



line indicates the continuity of the cavity of the stalk with 



that of the primary vesicle. 

 The line 0, 0, through which the section shewn in Fig. 50 C is 



supposed to be taken, passes through the choroidal fissure. 



