ORGANS OF VISION OF THE VERTEBRATA. 



399 



A. 



7,- 



superficial epiblast take place, that the front wall of the optic vesicle 

 is pushed close up to the hind Avail, 

 and the cavity of the vesicle becomes 

 almost obliterated (fig. 285 B). 



The bulb of the optic vesicle is 

 thus converted into a cup with double 

 walls, containing in its cavity the 

 portion of involuted epiblast. This 

 cup, in order to distinguish its cavity 

 from that of the original optic vesicle, 

 is generally called the secondary optic 

 vesicle. We may, for the sake of 

 brevity, speak of it as the optic cup; 

 in reality it never is a vesicle, since 

 it always remains widely open in 

 front. Of its double walls the inner 

 or anterior (fig. 285 B, r) is formed 

 from the front portion, the outer or 

 posterior (fig. 285 B, u) from the hind 

 portion of the wall of the primary 

 optic vesicle. The inner or anterior 

 (r), which very speedily becomes 

 thicker than the other, is converted 

 into the retina: in the outer or pos- 

 terior (w), which remains thin, pig- 

 ment is eventually 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, I). 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 vesic^le lies at the side of the head, and its stalk is directed 

 downwards, ijavvards and backwards. The stalk in fact slants away 



Fig. 285. Diagbammatic sectionfs 

 illustrating the formation of the 

 EYE. (After Eemak.) 



In A the thin superficial epihlast 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 u, 

 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 ii and an anterior wall r. 

 In the hollow of this cup lies the lens I, 

 now completely detached from the 

 superficial epiblast xh. 



