708 MANUAL OF PHYSIOLOGY. 



is obliterated. The two layers of the vesicle are now cup-shaped, 

 and receive the name of the secondary optic vesicle or the optic 

 cup. This ultimately becomes the retina, and the optic stalk 

 losing its cavity is transformed into the optic nerve. 



Meanwhile, the local involution of the epiblast over the optic 

 cup, which is the rudiment of the crystalline lens, becomes grad- 

 ually separated from the general epiblast giving origin to it, and 

 is finally detached from its point of origin. It now lies as a 

 somewhat spherical body in the cavity of the optic cup within 

 the superficial mesoblast, which has closed over it. 



FIG. 292. 



Section through the head of a chick at the third day, showing the origin of the lens, 

 a. Epiblast thickened at c, which is the point of origin of the lens. o. Optic vesicle. 

 V-L. Anterior cerebral vesicle. F 2 . Posterior cerebral vesicle. 



The secondary optic vesicle grows (except at its lower part, 

 just at the junction of the optic stalk), so as to deepen the optic 

 cup, which contains the rudimentary lens. At the lower part 

 an interval is left, which receives the name of the choroidal fissure. 

 Through this gap in the secondary optic vesicle the mesoblast 

 enters and separates the lens from the optic cup, and forms the 

 vitreous humor. 



The mesoblast surrounding the optic cup develops two cover- 

 ings of the eye, an outer fibrous capsule called the sclerotic coat, 



