ORIGIN OF THE LENS 131 



same character is caused to develop by hormones, and fails to appear 

 if the gland which secretes the chemical determinants is removed. 



No other current explanation of the lens is anything but lame. The 

 co-existence of a functional retina and a functionless lens in the larval 

 lamprey may mean, as Studnicka thought, that the lens existed in some 

 other status before the rest of the present eye evolved. Possibly how- 

 ever it means no more than does the precocious presence of function- 

 less muscles in an embryo before their nerves have grown out to connect 

 with them. No one would argue that this means that those muscles once 

 functioned without nervous control. 



/-/-^ 



Fig. 58 — Illustrating Schimkewitsch's theory. From Walls, after Schimkewitsch. 



a, hypothetical ancestral skin-eye, with erect retina and intrinsic 'retinal' lens rl. b, phylo- 

 genetic stage comparable with embryonic cup — the eye, originally dorsal, has swung laterally 

 and ventrad, becoming passively indented (by resistant tissue) to create the embryonic 

 fissure; the retinal lens is now uselessly located, c, final condition of the eye, with new 

 lens derived from the skin; it is from the site of the supposed original retinal lens that 

 new lenses may be regenerated if the skin-lens is removed in the young embryo or even 

 (salamanders) in the adult. 



Tretjakoff thought that the primitive optic cup was attached to the 

 skin and developed contractile elements there (which later became the 

 piscine retractor lentis muscle) for producing to-and-fro accommoda- 

 tory movements of the optic cup relative to the skin. The lens then 

 arose as a sort of callus in response to the continual pull of the muscle 

 cells (Fig. 57). But the lower fishes have no retractor lentis; and in any 

 case there would have been no need whatever of accommodation until 

 the lens had already appeared and become capable of forming a crisp 

 image. Tretjakoff also attempted to account for the fact that in sala- 

 manders whose lenses are removed, new lenses may regenerate from the 

 dorsal pupil margins. This has been explained more cleverly, if no more 

 properly, by Schimkewitsch (Fig. 58). 



Franz's theory is new and ingenious. He suggests that the lens 

 evolved, when the neural tube was just closing, in such a position as 



