EARLY THEORIES 



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Lankester, in 1880, suggested that the eye of the vertebrate is com- 

 parable with that of the 'tadpole' larva of certain of the lower chordates, 

 the Ascidia. Others interpreted this suggestion as one of true homology, 

 and a debate sprang up over whether the ascidian eye was a degenerate 



Fig. 46 — Illustrating the ascidian theory as originally conceived. 



(At a time when the ascidian lens was mistakenly believed to lie toward the brain cavity). 

 From Walls, after Jelgersma. 



a, ascidian eye, consisting of a retinal evagination of the brain wall and an internal lens, //. 



b, hypothetical transitional stage in which two lenses were present, one on either side of 

 the retina. 



c, vertebrate retina and definitive, 'outer' lens, derived from skin. 



Fig. 47 — Illustrating Balfour's theory. From Walls, after Parker. 



Patches of photosensory cells are shown in the successive positions which they are supposed 

 to have occupied before and after the evolution of the neural tube and the retinal evagin- 

 ations thereof. 



offshoot of the vertebrate organ or a primitive fore-runner thereof. Froriep 

 later decided that neither of these views could be true, for the retina of 

 the ascidian eye is not inverted; but he thought that both eyes might 

 have had common ancestry in a pair of dermal eyes (Figs. 46 and 48). 



