THE ECTODERM AND ITS DERIVATIVES 



Optic cup Lens Retina 



171 



Developing rods and cones Choroid fissure Pigmented layer 



View into the larval eye. Rana pipiens, photograph. 



The newly inverted cavity thus formed by the invagination of the 

 lateral wall of the optic vesicle is known as the optic cup. The rim 

 of the cup now exposes the pupil. The cup will invaginate further and 

 grow in thickness until it all but obliterates the original optic vesicle 

 (opticoel). The lateral (retinal) and the inner or mesial (pigmented) 

 layers of the optic cup are therefore brought into close proximity with 

 only a slit-like space remaining between, the remains of the original 

 opticoel. Ventrally this double-layered optic cup is connected with the 

 optic stalk. 



The three-dimensional aspect of these changes must be understood. 

 Transverse, sagittal, or horizontal (frontal) sections through the eye 

 will appear to be essentially alike. The cup is circular with only a 

 ventral cut-out where the inverted optic stalk is attached. If one could 

 look directly into such a developing eye, after removal of the lens, the 

 impression would be of a horseshoe-shaped cup, the pupil of the eye, 

 with a groove-like opening ventral. This central cavity is the optic cup 

 but ventral to it is the double-layered groove of the optic stalk, known 

 as the choroid fissure. 



The lens of the eye is formed from the superficial ectoderm by in- 

 vagination of the deeper or nervous layer of ectoderm opposite the 



