402 



SECONDARY OPTIC CUP. 



The complete differentiation of these two coats of the eye does 

 not however take place till a late period. 



The cavity of the original optic vesicle was left as a nearly obli- 

 terated space between the two walls of the optic cup. By the end of 

 the third day the obliteration is complete, and the two walls are in 

 immediate contact. 



The inner or anterior wall is, from the first, thicker than the 

 outer or posterior ; and over the greater part of the cup this contrast 

 increases with the growth of the eye, the anterior wall becoming 

 markedly thicker and undergoing changes of which we shall have to 

 speak directly (fig. 289). 



In the front portion however, along, so to speak, the lip of the 

 cup, anterior to a line which afterwards becomes the ora serrata, 

 both layers cease to take part in the increased thickening, accom- 

 panied by peculiar histological changes, which the rest of the cup is 

 undergoing. Thus a hind portion or true retina is marked off from 

 a front portion. 



The front portion, accompanied by the mesoblast which imme- 

 diately overlies it, is behind the lens thrown into folds, the 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 



Fig. 288. 



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

 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 Eemak.) 



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

 before ; /. the choroidal fissure. 



P. 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 iip to the lens below as well as above. 

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



black pigment takes place in 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 



