THE EMBRYOLOGY OF THE EYE. 9 



over the margin of the vesicle-wall and becomes 

 continuous with the anterior portion of the 

 vascular sheath of the lens, forming the pupil- 

 lary membrane. The epiblastic vesicle-wall 

 and mesoblastic sheet develop simultaneously 

 to form the iris. The pupillary membrane 

 together with the remainder of the vascular 

 sheath of the lens is absorbed before birth. 



As the vesicle-wall grows, it is thrown into 

 a fold near the equator of the lens, and this 

 fold becoming filled with mesoblastic tissue 

 forms the ciliary body. Further growth of 

 the vesicle in an equatorial direction produces 

 a number of meridional folds, which form the 

 ciliary processes. 



At this time the distal wall of the vesicle, 

 which in its posterior portion thickens to form 

 the retina, thins down in its anterior portion 

 to a single layer of cells. This layer is called 

 the pars ciliaris retinae in that portion which 

 covers the ciliary body, and becomes pigmented 

 and forms a portion of the uveal layer where 

 it covers the posterior surface of the iris. 



