240 



THE EYE IN EVOLUTION 



Figs. 242 to 247. — The Ontogenetic Development of the Lateral 



EYE of VeBTEBRATES. 



Fig. 242. 



Fig. 243. 



Fig. 244. 



Fig. 245. 



Fig. 246. 



Fig. 242. — The appearance of the foveolae opticse ( / ) on the dorsal 

 ectoderm of the cephahc (medullary) plate {m.p.). 



Fig. 243. — Invagmation of the surface ectoderm with the optical area 

 to form the primitive neural tube. 



Fig. 244. — Evagination of the primary optic vesicle. 



Fig. 245. — The commencement of secondary invagination of the neural 

 epithelium with thickening of the surface epithelium. 



Fig. 246. — Invagination of the surface epithelium. 



Fig. 247. — Detachment of the lens from the surface epithelium. 



Miiller's cells) and an outer zone of sensory cells, perhaps the linear 

 descendants of the ependymal cells (rods and cones). In this way the 

 strati I-! cation of cells with their accompanying system of interconnecting 

 neurones allows the appearance of a complex conducting and associating 

 appara 's. With very few exceptions the retina of Invertebrates is 



