ORIGIN OF THE RETINA 



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Fig. 56 — Origin of the retinae of the median and lateral eyes. After Walls. 



a, pro-vertebrate stage with photosensory ectoderm and with the nerve cord still a part of 

 the skin, b, b', alternative stages in the evolution of the neural tube, depending upon 

 whether one adheres to the 'solid' or 'hollow' doctrine, c, tubular nervous system formed, 

 but with ependymal lining purely sustentative, secretory, and circulatory, d, ependyma has 

 become photosensory locally, and photosensory cells have disappeared from the skin, 

 e, f, g, stages in the evolution of patches of photosensory ependyma into retinae. 



Origin of the Lens — When everything else in the primitive eye is so 

 plausibly explicable, it is really a shame that we cannot be at all sure how 

 the lens came into existence. The lens placode fits neatly into the set of 

 cephalic lateral-line organs, and for it to develop into a lens is no more 

 remarkable than for one of them to generate the olfactory organ or for 

 another of them, the otic placode, to differentiate into the elaborate 



