4 8 4 



PRIMARY OPTIC VESICLE. 



there is an account of the accessory structures connected with 

 the eye. 



The formation of the eye commences with the appearance of 

 a pair of hollow outgrowths from the anterior cerebral vesicle or 

 thalamencephalon, which arise in many instances, even before 

 the closure of the medullary canal. These outgrowths, known 

 as the optic vesicles, at first open freely into the cavity of the 

 anterior cerebral vesicle. From this they soon however become 

 partially constricted, and form vesicles (fig. 284, a], united to the 

 base of the brain by compara- 

 tively narrow hollow stalks, the 

 rudiments of the optic nerves. 

 The constriction to which the 

 stalk or optic nerve is due takes 

 place obliquely downwards and 

 backwards, so that the optic 

 nerves open into the base of the 

 front part of the thalamencephalon 

 (fig. 284, b}. 



After the establishment of the 

 optic nerves, there take place (i) 

 the formation of the lens, and (2) 

 the formation of the optic cup 

 from the walls of the primary optic vesicle. 



The external or superficial epiblast which covers, and is in 

 most forms in immediate contact with, the most projecting 

 portion of the optic vesicle, becomes thickened. This thickened 

 portion is then driven inwards in the form of a shallow open 

 pit with thick walls (fig. 285 A, o\ carrying before it the front 

 wall (r) of the optic vesicle. To such an extent does this 

 involution of the superficial epiblast take place, that the front 

 wall of the optic vesicle is pushed close up to the hind wall, and 

 the cavity of the vesicle becomes almost obliterated (fig. 285 B). 



The bulb of the optic vesicle is thus converted into a cup 

 with double walls, containing in its cavity the portion of 

 involuted epiblast. This cup, in order to distinguish its cavity 

 from that of the original optic vesicle, is generally called the 

 secondary optic vesicle. We may, for the sake of brevity, speak 

 of it as the optic cup; in reality it never is a vesicle, since it 



FIG. 284. SECTION THROUGH 

 THE HEAD OF AN EMBRYO TELEOS- 

 TEAN, TO SHEW THE FORMATION OF 



THE OPTIC VESICLES, ETC. (From 



Gegenbaur ; after Schenk.) 



c. fore-brain ; a. optic vesicle ; b. 

 stalk of optic vesicle ; d. epidermis. 



