ORGANS OF VISION OF THE VERTEBRATA. 



403 



seems to become nothing- more than a forward prolongation of the 

 pigment epithelium of the choroid. 



Thus, while the hind moiety of the optic cup becomes the retina 

 proper, including the choroid-pigment in which the rods and cones 



c I 



p.Cf, 



FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAT. 



e.p. superficial epiblast of the side of the head ; R. true retina: anterior wall of the 

 optic cup; p.Ch. pigment-epithelium of the choroid: posterior wall of the optic cup. 

 b is placed at the extreme lip of the optic cup at what will become the margin of the 

 iris. /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at nl, 

 now forms nearly the whole mass of the lens, the front wall being reduced to a layer of 

 flattened cells el. in. the mesoblast surrounding the optic cup and about to form the 

 choroid and sclerotic. It is seen to pass forward between the lip of the optic cup and 

 the superficial epiblast. 



Pilling up a large part of the hollow of the optic cup is seen a hyaline mass, the 

 rudiment of the hyaloid membrane, and of the coaguhim of the vitreous humour, y. In 

 the neighbourhood of the lens it seems to be continuous as at cl with the tissue a, which 

 appears to be the rudiment of the capsule of the lens and suspensory ligament. 



are imbedded, the front moiety is converted into the ciliary portion 

 of the retina, covering the ciliary processes, and into the uvea of the 

 iris ; the bodies of the ciliary processes and the substance of the iris, 

 their vessels, muscles, connective tissue and ramified pigment, being 

 derived from the mesoblastic choroid. The margin of the pupil marks 

 the extreme lip of the optic vesicle, where the outer or posterior wall 

 turns round to join the inner or anterior. 



The ciliary muscle and the ligamentum pectinatum are both 

 derived from the mesoblast between the cornea and the iris. 



20 2 



