THE LATER DEVELOPMENT OF THE CHICK 319 



more important events in the further history of the structures. 



The optic cup extends ventrally as well as in other directions, 

 so that the attachment of the optic stalk appears no longer 

 connected with the ventral side, but rather with the middle of 

 the proximal hemisphere, nearly opposite the pupil and lens; 

 this is known as the fundus region. Associated with this 

 change is a decided alteration in the character and relations of 

 the choroid fissure, which will be described below. 



About the sixth or seventh day the thick inner layer of the 

 optic cup becomes clearly differentiated into a proximal or 

 retinal zone, and a distal or lenticular zone. The former in- 

 cludes rather more than the proximal hemisphere, while the 

 latter forms a broad band bordering the pupil. The retinal 

 portion becomes transformed later into the sensory and nervous 

 parts of the eye; the thin outer layer in the retinal zone of the 

 cup is not sensory, but is transformed into the pigmented 

 layer of the retina. Both the inner and outer layers of the cup 

 in the lenticular zone share in forming the rudiments of the 

 iris and ciliary process. The separation between the retinal 

 and lenticular zones is marked in the developed eye by the 

 ora serrata. 



In the lenticular zone the thin outer layer fuses with the 

 inner layer, becomes pigmented, and together with mesenchyme 

 cells of the region they form the iris (Fig. 127, D). The proper 

 muscles of the iris are not derived from the mesenchyme, but 

 from bud-like outgrowths of the lenticular zone (ectoderm). 

 The ciliary process is at first a series of folds of the lenticular 

 zone around the base of the iris, but soon these folds are 

 invaded by mesenchyme cells which form the muscles of the 

 ciliary process. 



During the expansion of the optic cup the margins of the 

 choroid fissure have come into close apposition, nearly closing 

 the fissure. Owing to the originally ventral attachment of 

 the optic stalk the retinal layer is continuous, either side of the 

 fissure, with the lower side of the stalk, and the connection 

 extends through about one quadrant of the optic cup, i.e., 

 from the fundus nearly around to the margin of the retinal 



