CHAPTER 3 



TYPES OF VERTEBRATE AND INVERTEBRATE 



EYES 



By the study of the adult organ and different stages of its development 

 in the various types of eye — " upright " or " inverted," " simple " or 

 " compound," " single " or " composite " — it has been shown that the 

 receptive or sensory cells of all these types have been evolved by modifica- 

 tion of the surface layer of epithelium. In invertebrates the epithelium is 

 derived from the " hypoderm " which lies beneath the cuticle (Fig. 4). 



Fig. 4. — Section through the Ocellus of a Young Dytiscus Larva. 



Grenacher.) 



ct. : chitinous cuticle. rh. : rods. 



vc. : cells of the vitreous body. re. : retinal cells. 



/. : cuticular lens. n.o. : optic nerve. 

 hy. : hypodermis. 



(After 



In vertebrates the sensory epithelium originates from the medullary 

 plate, or " neural ectoderm." This is at first spread out on the surface 

 of the embryo and forms an elongated placode of thickened epithelium 

 continuous at its edges with the surrounding cutaneous ectoderm, and in 

 this respect it resembles the olfactory, otic, and branchial placodes 



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