SENSE OF SIGHT. 179 



rain (Fig. 153). It will thus be seen that the optic nerves and 

 esieles are of the same origin as the cerebral vesicle— both repre- 

 ssing parts of the wall of the fore-brain. 



(5) The pigmentary layer of the retina is formed from the 

 asheathing or outer layer of the optic cup (Fig. 146). At 

 rst the wall of the optic vesicle is composed of a single layer of 

 pithelium ; the outer or pigmentary layer of the retina retains 

 lis embryonic form. 



uvea 



lens, optic, cup 



-' optic, stalk-. 

 or nerve j 



pars ciliar. ret 

 cavity of vesic. 



outer or pigment layer. inner or inuag. layer 



Fig. 146. — Diagrammatic Section of the Optic Cup and Lens. 



(c) The uvea is the layer of pigmented epithelium which covers 

 le posterior surface of the iris. It is formed out of both outer 

 id inner layers of the optic cup, and represents the rim of the 

 jp (Fig. 146). 



(d) The Pars ciliaris retinae is formed out of that part of the 

 mer or invaginated layer of the optic cup which lies in the 

 ladow of the iris, and is therefore inaccessible to light rays. 

 ; also retains the primitive columnar form of the epithelium. 

 he ora serrata marks the junction of the pars ciliaris retinae 

 id sensitive retina. 



Ciliary Processes. — At the commencement of the third month, 

 le pars ciliaris retinae becomes plicated or puckered into 60 or 

 small folds ; mesoblast of the middle tunic (choroid) grows 

 to the puckers and forms the ciliary processes. It should be 

 iserved that the lens lies within the optic cup and the ciliary 

 ■ocesses are formed round the equator or circumference of the 



QS. 



(«) The Sensitive Retina is formed out of the inner or in- 

 iginated layer of the optic cup (Fig. 146). At first the inner 



