VI.] THE EYE. 137 



of the stalk leading away from the almost obliterated 

 cavity of the primary vesicle. 



Fig. 48 represents the early stage at which the 

 lens fills the whole cup of the secondary vesicle. The 

 subsequent state of affairs is brought about through 

 the growth of the walls of the cup taking place more 

 rapidly than that of the lens. 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 part of the circumference of the cup which 

 adjoins 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 cho- 

 roidal fissure, a way is open from the mesoblastic tissue 

 surrounding the optic vesicle and stalk into the interior 

 of the cavity of the cup. 



From the manner of its formation the gap or fissure 

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

 and in order to be seen must be looked for on the 

 under surface of the optic vesicle. In this position it 

 is readily recognized in the transparent embryo of the 

 third day, Figs. 37 and 48. 



Bearing in mind these relations of the gap to the 

 optic stalk, the reader will understand how sections of 

 the optic vesicle at this stage present very different 

 appearances according to the plane in which the 

 sections are taken. 



When the head of the chick is viewed from under- 

 neath as a transparent object the eye presents very 



