4 8 



THE PINEAL ORGAN 



Development of the Lens and other Refractile Elements of the Eye 



These may be derived from : 



i. The cutaneous ectoderm or epidermis, including the cuticle and 

 the hypoderm cells (Figs. 4, 9, and 10). 



2. The superficial epithelial stratum of the retina, by a special modifica- 



•op.n. 



Fig. 33. — Simple Optic Pit of a Limpet (Patella). 



The pit is lined by pigmented epithelium, which is continuous at the margin of the 

 pit with the cuticular epithelium. The outer ends of the cells are clear and 

 rod-like. The nuclei are deep to the pigment layer, and the inner ends of 

 the cells are continued into the fibres of the optic nerve, op. n. : optic 

 nerve ; r. : retina. 



(Cambridge Natural History, after Helger.) 



op.n. 



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Fig. 34. — Open Optic Vesicle of Trochus. 



The cavity of the vesicle is filled with a semi-fluid secretion, the vitreous humour, 

 v.h. ; op. n., optic nerve ; pig., pigment. 



tion of certain cells or parts of cells belonging to the retinal segment of the 

 optic cup or optic vesicle (Figs. 33, 36). 



3. The distal or superficial wall of the optic vesicle — hypoderm cells 

 in invertebrates or neural ectoderm in vertebrates (Figs. 37, 10). 



