VI.] THE EYE. 185 



Diagrammatic Sections illustrating the Formation or 

 THE Eye. (After Eemak.) 



In A, the thin superficial epiblast h is seen to be thickened at x, 

 in front of the optic vesicle, and involuted so as to form 

 a pit o, the mouth of which has already begun to close in. 

 Owing to this involution, which forms the rudiment of the 

 lens, the optic vesicle is doubled in, its front portion r being 

 pushed against the back portion u, and the original cavity 

 of the vesicle thus reduced in size. The stalk of the vesicle 

 is shewn as stUl broad. 



In B, the optic vesicle is stUl further doubled in so as to form a 

 cup with a posterior wall u and an anterior wall r. In the 

 hollow of this cup lies the lens I, now completely detached 

 from the superficial epiblast x. Its cavity is still shewn. 

 The cavity of the stalk of the optic vesicle is already much 

 narrowed. 



blast, which forms a continuous layer in front of it, 

 all traces of the original opening being lost. There is 

 thus left lying in the cup of the secondary optic vesicle, 

 an isolated elliptical mass of epiblast. This is the 

 rudiment of the lens. The small cavity within it 

 speedily becomes still less by the thickening of the 

 walls, especially of the hinder one. 



At its first appearance the lens is in immediate 

 contact with the anterior wall of the secondary optic 

 vesicle (Fig. 47 B). In a short time, however, the lens 



