486 



CHOROID FISSURE. 



slants away from the vesicle. Hence, when the involution of 

 the lens takes place, the direction in which the front wall of the 

 vesicle is pushed in is not in a line with the axis of the stalk, 

 as for simplicity's sake has been represented in the diagram 

 (fig. 285), but forms an obtuse angle with that axis, after the 

 manner of fig. 286, where / represents the cavity of the stalk 

 leading away from the almost obliterated cavity of the primary 

 vesicle. 



Fig. 286 represents the early stage at which the lens fills the 

 whole cup of the secondary vesicle. The subsequent condition 

 is brought about through the rapid 

 growth of the walls of the cup. This 

 growth however 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 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 choroidal 



fissure, a way is open from the meso- 



blastic tissue surrounding the optic 



vesicle and stalk into the interior of the 



cavity of the cup. 



From the manner of its formation 



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 recognised in the embryo 



seen as a transparent object (fig. 118, chs). 



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 is viewed from underneath as a transparent 



FIG. 286. DIAGRAMMATIC 

 SECTION OF THE EYE AND 

 THE OPTIC NERVE AT AN 

 EARLY STAGE. (From Lie- 

 berkiihn.) 



To shew the lens / occu- 

 pying the whole hollow of 

 the optic cup, the inclination 

 of the stalk s to the optic 

 cup, and the continuity of the 

 cavity of the stalk s' with that 

 of the primary vesicle c ; r. 

 anterior, u. posterior wall of 

 the optic cup. 



the 



gap 



or fissure is 



