ORGANS OF VISION OF THE VERTEBRATA. 489 



ciliary ridges ; while further forward it bends in between the 

 lens and the cornea to form the iris. The original wide opening 

 of the optic cup is thus narrowed to a smaller orifice, the pupil ; 

 and the lens, which before lay in the open mouth of the cup, is 

 now inclosed in its cavity. While in the hind portion of the 

 cup or retina proper no deposit of black pigment takes place in 



D 



E 

 A- 



FIG. 288. 



D. Diagrammatic section taken perpendicular to the plane of the paper, along 

 the line yy, fig. 287. The stalk is not seen, the section falling quite out of its region. 

 vh. hollow of optic cup filled with vitreous humour ; other letters as in fig. 285 B. 

 (After Remak.) 



E. Section taken parallel to the plane of the paper through fig. 287, so far behind 

 the front surface of the eye as to shave off a small portion of the posterior surface of 

 the lens /, but not so far behind as to be carried at all through the stalk. Letters as 

 before ; f. the choroidal fissure. 



F. Section along the line zz, perpendicular to the plane of the paper, to shew the 

 choroidal fissure/, and the continuity of the cavity of the optic stalk with that of the 

 primary optic vesicle. Had this section been taken a little to one side of the line zz, 

 the wall of the optic cup would have extended up to the lens below as well as above. 

 Letters as before. The external epiblast is omitted in this section. 



the layer formed out of the inner or anterior wall of the vesicle ; 

 in the front portion forming the region of the iris, pigment is 

 largely deposited throughout both layers, though first of all in 

 the outer one, so that eventually this portion seems to become 

 nothing more than a forward prolongation of the pigment epi- 

 thelium 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 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 



