400 



CHOROI1) FIXTURE. 



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 s represents the 

 cavity of the stalk leading away from the almost obliterated cavity 

 of the primary vesicle. 



Fig. 2Sfj 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 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 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 object 

 the eye presents very much the appearance represented in the dia- 

 gram "(fig. 287). 



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. 288 D. The lens, the cavity and double walls of the secondary 

 vesicle, the remains of the primary cavity, would all be represented 

 (the superficial epiblast of the head would also be shewn); but there 

 would be nothing seen of either the stalk or the fissure. If on the 

 other hand the section were taken in a plane parallel to the plane of 

 the paper, at some distance above the level of the stalk, some such 

 figure would be obtained as that shewn in fig. 288 E. Here the 



FIG. 286. DIAGRAMMATIC 

 SECTION OF THE EYE AND THK 

 OPTIC NERVE AT AN EARLY 

 STAGE. (From Lieberkiihn.) 



To shew the lens I 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. 



