154 THE THIRD DAY. [CHAP. 



receives its final form upon the full development of the 

 iris. 



Summary. We may briefly recapitulate the main 

 facts in the development of the eye as follows. 



The eye commences as a lateral outgrowth of the 

 fore-brain, in the form of a stalked vesicle. 



The stalk, becoming narrowed and subsequently 

 solid, is converted into the optic nerve. 



An involution of the superficial epiblast over the 

 front of the optic vesicle, in the form first of a pit, then 

 of a closed sac with thick walls, and lastly, of a solid 

 rounded mass (the small central cavity being entirely 

 obhterated by the thickening of the hind wall), gives 

 rise to the lens. Coincidently with this involution of 

 the lens, the optic vesicle is doubled up on itself, and 

 its cavity obliterated ; thus a secondary optic vesicle 

 or optic cup with a thick anterior and a thin posterior 

 wall is produced. As a result of the manner in which 

 the doubhng up takes place, or of the mode of growth 

 afterwards, the cup of the secondary optic vesicle is at 

 first imperfect along its under surface, where a gap, the 

 choroidal fissure, exists for some little time, but subse- 

 quently closes up. 



The mesoblast in which the eye is imbedded gathers 

 itself together around the optic cup into a distinct in- 

 vestment, of which the internal layers become the 

 choroid, the external the sclerotic. An ingrowth of 

 this investment between the front surface of the lens 

 and the superficial epiblast furnishes the body of the 

 cornea, the epiblast itself remaining as the anterior 

 corneal epithelium. 



The mesoblast entering on the under side through 



