50 THE EMBRYOLOGY OF THE EYE. 



situation it at first retains its original thick- 

 ness. A little later its margin becomes 

 thinner (Fig. 9), and still later (Figs. 11, 13) 

 we find nothing left of this mesoblastic out- 

 growth running to the vascular sheath of the 

 lens but the endothelial laver, and a few 

 spindle-cells from the anterior layer, which 

 accompany vessels. The spindle-cells extend 

 only a short distance into the pupillary mem- 

 brane, and in its central portion the membrane 

 consists only of a single layer of capillaries, 

 much smaller than those which lie posterior 

 to the iris and carry the blood from the vessels 

 in the vitreous. 



At a late stage the mesoblastic margin of 

 the iris extends out some distance beyond the 

 farthest iris vessels, and capillaries springing 

 from vessels of the minor zone pass from the 

 anterior surface of the iris, some millimetres 

 from its margin, and run to the pupillary 

 membrane. The membrane disappears just 

 before birth, or just after birth in some ani- 

 mals, and usually there is no trace of it left. 



