THE PLACODE THEORY; BOVERl'S THEORY 



The Placode Theory — The origin of the lens was first explained by 

 Sharp in 1885. He regarded the lens as a modified lateral-line organ 

 which was, like those organs, a sensory ectodermal pit or bud. The 

 'placode theory', an extension of Sharp's original idea, proposes that 

 the lens was once the whole eye and that the present retina served as its 

 ganglion, eventually taking over the sensory function itself and releasing 

 the vesicular 'skin' eye to become a lens (Fig. 51). Fatal objections to 

 this interpretation of the retina arise from the utter absence of embry- 



Fig. 53 — Illustrating Boveri's theory. From Walls, after Boveri. 

 The Hesse's organs become the visual and pigment-epithelial cells of the vertebrate retina. 



©logical confirmation of any previous connection of retina and lens, 

 and from the lack of any evidence that a self-determining lens placode 

 exists at all as a morphological entity — it will be recalled that it is called 

 into existence ontogenetically solely by the chemical influence of the 

 optic cup. Nor does the placode theory account for inversion. 



Boveri's Theory — Inversion was explained anew by Boveri in 1904, 

 in a theory that made use of the two-celled visual organs of Amphioxus, 

 which had been discovered by Hesse in the 'spinal cord' of this so-called 

 grandfather of the vertebrates (Figs. 52 and 53). While Boveri's theory 



