v.] 



THE CHOROIDAL FISSURE. 



99 



walls of the cnp taking place more rapidly than that of the 

 lens, or in other words to the cavity of the cup dilating. 

 But this growth or this dilatation does not take place equally 

 in all parts of the cup. The walls of the cup rise up all 

 round except that point of the circumference of the cup 

 which is opposite the middle (from side to side) of the stalk. 

 While elsewhere the walls increase rapidly in height, carrying 

 so to speak the lens with them, at this spot, which in the 

 natural position of the eye is on its under surface, there is 

 no growth : the wall is here imperfect, and a gap is left. 

 Through this gap, which afterwards receives the name of the 

 choroidal fissure, a way is open from the mesoblastic tissue 

 surrounding the optic vesicle and stalk into the interior of 

 the cavity of the cup. 



Fig. -29. 



Diagrammatic EErnESBNTATioN of the Eye of the Chick of about the 



THIRD DAT AS SEEN WHEN THE HEAD IS VIEWED EEOM UNDERNEATH AS 

 A TRANSPARENT OBJECT. 



/ the lens, V 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 tlie 

 piimary optic vesicle, now nearly obliteiated. By inadvertence u has been 

 drawn in some places thicker than r, it should have been thinner through- 

 out. 



X 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 z, s, tlirough which the section shewn in Fig. 30 F is supposed 



to be taken, passes through the choroidal fissure. 



From the manner of its formation the gap or fissure is 

 evidently in a line with the axis of the optic stalk, and in 



7—2 



