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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



terior or epithelial layer of the back-folded nightcap 

 becomes the bacillary layer of the retina, while the 

 anterior layer of the same is the fibrous layer of the 

 retina. 



The difference between the vertebrate and inverte- 

 brate retina is great, but not so great as at first seems. 

 In both cases the bacillary layer is formed of epithelial 

 cells — in the one of exterior epithelium of skin, in the 

 other of interior epithelium of the brain. But the lining 

 epithelium of the brain is itself an infolded portion of 

 the epiderm (Fig. 108, A and B). 



Transition from Invertebrate to Vertebrate 

 Eye. — If vertebrates came from some form of inverte- 

 brates, as undoubtedly they did, how was the vertebrate 

 eye evolved out of the invertebrate eye ? This is a very 

 difficult question, i. Of course, the vertebrate type of 

 eye must have branched off very low down and before 

 the invertebrate type was fully declared. 2. Much of the 

 difficulty has come of identifying the crystalline lens of 

 the vertebrate eye with the same of the invertebrate 

 eye. On the contrary, it corresponds to the whole eye 

 of the invertebrates. It is formed in the same way, 

 viz., by infolding of the epidermal surface, while the 

 lens of invertebrates is formed by cuticular ingrowth 

 from the corneal surface. The retina of the verte- 

 brate eye is something superadded to the whole eye of 

 invertebrates, the retinal part of the latter having been 

 aborted and modified to form the back part of the ver- 

 tebrate lens. 



Thus much seems certain, but how the change came 

 about is obscure. We may imagine (a) some low form 

 of invertebrate with very imperfect invertebrate eye, the 

 infolded epiderm functioning as usual as retina, but this 

 very close to cephalic ganglion. The light stimulating 

 the cephalic ganglion might well provoke the formation 



