106 



THE GENESIS OF THE VERTEBRATE EYE 



formation of the lens vesicle is absolutely dependent upon the presence 

 of the optic vesicle against the surface ectoderm — but not in any 

 mechanical way: the lens-organizing influence of the optic vesicle is 

 exerted chemically. If the optic vesicle is removed, no lens vesicle is 



Fig. 38 — Formation of the optic cup. 



a, b, c, diagrammatic models of optic vesicle, transitional stage, and completed cup as seen 

 from the side of the embryonic head with the surface ectoderm removed. The curved arrows 

 in b show the direction of growth of the lateral portions of the vesicle which, while the 

 indentation of the face of the vesicle is taking place, grow below the level of the axis of the 

 optic stalk (dotted line) to form the ventral half of the cup. The embryonic fissure is 

 created by the temporary failure of the down-growing lobes to meet and fuse. 



a', b', c', optical sections through the stalk axis (dotted line), corresponding respectively 

 to a, b, and c. A patch of surface eaoderm has been left in place to show the development 

 of the lens vesicle. 



ef- embryonic fissure of optic cup; g- groove on underside of optic stalk (continuation of 

 embryonic fissure); i- invagination of face of vesicle; il- inner layer of optic cup (future 

 retina); Ip- lens placode; h- lens vesicle; ol- outer layer of optic cup (future pigment 

 epithelium); s- stalk; se- surface ectoderm. 



formed; and if the optic vesicle is planted under any other surface ecto- 

 derm, even on the belly of the embryo, a lens vesicle will proceed to 

 form from that ectoderm. Similarly, if the developing lens vesicle is 

 removed, the optic vesicle goes right ahead with its indentation — the 

 latter is an active process, not caused mechanically by the inward pres- 



