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THE GENESIS OF THE VERTEBRATE EYE 



Balfour's Theory — It was Balfour, in 1881, who first proposed that 

 the vertebrate retina originated in the skin and was carried inside the 

 animal by the evolution of the neural tube (Fig. 47). Several investi- 

 gators, independently of each other, soon pointed out how well the fove- 



Fig. 48 — Illustrating Froriep's derivation of the ascidian and vertebrate eyes. 

 (From common-ancestral superficial vesicular eyes). After Walls. 



a, b, c, d, stages in the evolution of the ascidian eyes, showing the degeneration of one 

 member of the pair. 



a, b', c', d', stages in the evolution of the lateral eyes of vertebrates. 



o\x Opticas fit into this hypothesis (Figs. 49 and 50). Balfour's theory- 

 was the first to account for the inversion of the retina, but it offered no 

 explanation of the lens. It has however been suggested that inversion was 

 no accident, but had to be brought about somehow if the highly meta- 



