8 THE EMBRYOLOGY OF THE EYE. 



the internal or distal layer becoming thicker 

 to form the retina, and the external or proxi- 

 mal layer diminishing to a single stratum of 

 cells, which become pigmented and form the 

 pigment-epithelium. 



While this differentiation is taking place, 

 the mesoblastic tissue just about the vesicle 

 becomes denser and many blood-vessels ap- 

 pear in it, forming the rudiment of the choroid. 



Mesoblastic tissue also pushes out just be- 

 hind the external epiblastic layer and forms 

 the rudimentary cornea, the external epiblastic 

 layer forming the epithelium. A single layer 

 of unbranched mesoblastic cells covering the 

 primitive cornea posteriorly becomes the 

 corneal endothelium, and this secretes Desce- 

 met's membrane. 



In the meantime the secondary vesicle grows 

 much faster than the lens, and the folded 

 margins of the vesicle-wall which remain in 

 contact with the lens are drawn inward, thus 

 beginning the formation of the iris. A sheet 

 from the surrounding mesoblast passes out 



