42 Vertebrate Embryology 



very young tadpoles as an evagination from 

 each side of the fore-brain. This evagination, 

 which is known as the optic vesicle, reaches a 

 considerable size and becomes constricted off 

 from the brain, so that it forms a large, hollow 

 bulb connected with the brain by a very nar- 

 row stalk. 



The walls of the optic vesicle are at first 

 comparatively thin, but as the vesicle enlarges 

 and approaches the superficial ectoblast, the 

 wall of the vesicle that is next to the ectoblast 

 begins to thicken and at the same time to be 

 pushed in on itself (Fig. 19, OC}] this invagi- 

 nation of the optic vesicle continues until the 

 two walls are in contact, and the cavity of the 

 original optic vesicle is obliterated. The cup- 

 shaped structure which is thus formed, and 

 which is still connected with the fore-brain by 

 the narrow stalk, is known as the optic cup. 

 The thick inner wall of the optic cup forms 

 the essential part of the retina, while the thin 

 outer wall forms the pigmented layer that sur- 

 rounds the retina. The rim of the optic cup 

 is not complete, like the rim of an invaginated 

 hollow rubber ball, but is interrupted, at one 

 place, as a narrow slit known as the choroid 

 fissure. Through the choroid fissure the sur- 



