THE EYE. 711 



hemispheres and that of the thalamencephalon, which becomes 

 the third ventricle. Later, the optic vesicles open directly into 

 the third ventricle, and finally are displaced backwards, and 

 come into connection with the mid -brain. 



The optic vesicles are at first hollow prolongations, which con- 

 sist of an anterior dilated portion, forming the primary optic 

 vesicle, and a posterior tubular portion or stalk joining the vesicle 

 to the fore-brain. This stalk forms the optic nerve. 



As each vesicle grows forwards towards the epiblast covering 

 the head of the embryo, the epiblastic cells at the spot overlying 

 the vesicle become thickened, and an involution of the epiblast 

 takes place towards the optic vesicle, and indents the latter, 

 approximating its anterior to its posterior wall. 



By this means the anterior and posterior walls of the primary 

 optic vesicle come into close contact, and the cavity of the vesicle 

 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. 



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, 

 and a vascular coat, the choroid, which lies in contact with the 

 outer layer of the optic cup. 



In front of the lens, beneath the epiblast, the mesoblast forms 



