ORGANS OF VISION OF THE VERTEBRATA. 



485 



Of its double walls 



A. 



ft 



always remains widely open in front, 

 the inner or anterior (fig. 285 

 B, r) is formed from the front 

 portion, the outer or posterior 

 (fig. 285 B, #) from the hind por- 

 tion of the wall of the primary 

 optic vesicle. The inner or ante- 

 rior (r), which very speedily be- 

 comes thicker than the other, is 

 converted into the retina : in 

 the outer or posterior (?/), which 

 remains thin, pigment is even- 

 tually deposited, and it ultimately 

 becomes the tesselated pigment- 

 layer of the choroid. 



By the closure of its mouth 

 the pit of the involuted epiblast 

 becomes a completely closed sac 

 with thick walls and a small 

 central cavity (fig. 285 B, /). At 

 the same time it breaks away 

 from the external epiblast, 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. 285 B). In 

 a short time however, the lens is seen to lie in the mouth of the 

 cup (fig. 288 D), a space (vh] (which is occupied by the vitreous 

 humour) making its appearance between the lens and anterior 

 wall of the vesicle. 



In order to understand how this space is developed, the 

 position of the optic vesicle and the relations of its stalk must 

 be borne in mind. 



The vesicle lies at the side of the head, and its stalk is 

 directed downwards, inwards and backwards. The stalk in fact 



FIG. 285. DIAGRAMMATIC SEC- 

 TIONS ILLUSTRATING THE FORMATION 

 OF THE EYE. (After Remak.) 



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. Accompany- 

 ing 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 tt, and 

 the original cavity of the vesicle thus 

 reduced in size. The stalk of the vesicle 

 is shewn as still broad. 



In B the optic vesicle is still 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 /, 

 now completely detached from the 

 superficial epiblast xh. 



