CHAPTER XIV. 

 THE EYE. 



The eyes begin to develop during the fourth week 

 of embryonic life, and appear then as a pair of lateral 

 e vagina tions of the fore-brain. A pair of vesicles 

 are thus formed called the primary optic vesicles. 

 When the latter reach the ectoderm an invagination 

 of these vesicles takes place, like pushing in one 

 side of a hollow rubber ball. The cavity of the 

 primary optic vesicle becomes obliterated by this 

 process, and a new vesicle forms, called the secondary 

 optic vesicle. It will be observed that the cavity of 

 this vesicle is practically the same as would be pro- 

 duced by an invagination of the brain wall. Later 

 it will be seen that this cavity corresponds to the 

 space occupied by the vitreous humor of the adult 

 eye, while its wall becomes the retina. The stalk 

 that connects this vesicle to the brain is the optic 

 stalk, in which later optic nerve fibers appear. 



At the time the secondary optic vesicle is forming 

 there is a disc-like thickening of the adjacent ecto- 

 derm, which soon invaginates and becomes con- 

 stricted as an ectodermal vesicle. This is the lens, 

 which later takes a position at the mouth of the 

 secondary optic vesicle. The latter presents, at this 

 stage, a fissure in its ventral surface called the cho- 

 roid fissure. Connective-tissue cells migrate through 



