DEVELOPMENT OF THE EYE. 



447 



forming a double-walled cup the secondary optic vesicle, or optic 

 cup. An internal and an external wall may now be differentiated, 

 continuous at the margin of the cup. At the same time a disc-like 

 thickening of the adjacent ectoderm sinks inward toward the mouth 

 of the cup-shaped optic vesicle, forming the first trace of the lens. 

 During the development of the secondary optic vesicle a groove 



Blood-vessels Sphincter 



Vein. Canal of Petit, of the iris. Cornea, pupillae. Iris. 



Fontana's spaces. 



Pigment 

 layer. 



Physiologic excavation. 



Macula lutea. 



Fig. 35 2 . Schematic diagram of the eye (after Leber and Flemming) : a, Vena vorti- 

 cosa ; b, choroid ; /, lens. 



is formed on its ventral side, extending from the marginal ring into 

 the optic stalk. This is the embryonic optic fissure, or the choroi- 

 dal fissure. At the edges of the groove the two layers of the optic 

 cup are continuous. This groove serves for the penetration of 

 mesoblastic tissue and blood-vessels into the interior of the optic 

 cup, and in its wall the fibers of the optic nerve develop. 



The outer layer of the secondary optic vesicle becomes the pig- 

 ment membrane ; the inner, the retina. The optic nerve-fibers con- 

 sist not only of the centripetal neuraxes of certain ganglion cells in 

 the retina, but also of centrifugal neuraxes, which pass out from 

 the brain (Froriep). 



The invaginating ectoderm which later constitutes the lens is 

 constricted off from the remaining ectoderm in the shape of a vesi- 



