THE EMBRYOLOGY OF THE EYE. 7 



tissue passes through it into the cavity of the 

 secondary vesicle. Previously a small quan- 

 tity of mesoblastic tissue has been carried in 

 with the lens and takes part in the formation 

 of its vascular sheath, but a greater quantity 

 of mesoblastic tissue passes through the in- 

 ferior cleft and forms the vitreous, which is 

 an almost structureless substance containing 

 blood-vessels. The vessels entering first at 

 the posterior extremity of the cleft divide 

 into two systems of branches, one of which 

 runs forward in the vitreous near the wall of 

 the cavity, and another runs to the posterior 

 pole of the lens where it breaks up into in- 

 numerable twigs which lie in the vascular 

 sheath of the lens. 



The cleft in the optic stalk closes about 

 these mesoblastic vessels and they come to lie 

 in the centre of the optic papilla, the branches 

 which run in the vitreous being obliterated 

 before birth. 



As soon as the secondary optic vesicle is 

 formed, its two layers begin to differentiate. 



