276 INTERNAL FACTORS IV. 2 



the lens may still be formed. It may also be mentioned that 

 Mencl has described a double-headed monster of Salmo salar in 

 which one head has two well-formed lenses, though the other 

 parts of the eye are absent. The lenses, however, lie quite close 

 to the fore-brain, which may have exerted the stimulus, supposing 

 a stimulus to be necessary. 



It will be seen that the evidence is extremely contradictory, 

 and forbids any definite conclusion. 1 



It is curious that in tadpoles grown in certain solutions 

 (sodium chloride, bromide, and nitrate) the optic cup is often at 

 some distance from the ectoderm, and that in these cases the lens 

 is absent, though it may happen that the upper edge of the 

 optic cup is sharply bent down in front of the aperture, as 

 though to form a lens from the edge of the iris, as occurs in the 

 regeneration of the lens, after extirpation, in the Newt. 2 



Spemann and Lewis have also found that in the absence of 

 contact between the optic cup and the ectoderm the cornea is 

 not developed, that is to say, the overlying ectoderm does not 

 ' clear ' (lose its pigment), it does not thin out, and Descemet's 

 membrane is not formed. The more extensive series of experi- 

 ments is due to Lewis, who turned back a flap of skin and 

 wholly or partially removed the optic vesicle, the skin being then 

 returned to its place. In the first case there was no eye, no lens, 

 no cornea, though on the other side of the body all were present. 

 In the second case the lens and cornea were both formed, 

 provided that enough was left of the vesicle for the optic cup 

 to come into contact with the ectoderm. As a control experi- 

 ment the flap of skin was merely removed and replaced; the 

 development of the eye was normal. If at a later stage, when 

 corneal changes had already begun, both optic cup and lens were 

 taken out, the cornea was not completed. The pigment indeed 

 disappeared, but the ectoderm did not diminish in thickness and 

 Descemet's membrane was not formed. 



Removal of the lens alone did not interfere with the production 



1 Stockard has recently shown that when the egg of Fundulus is placed 

 in MgCl 2 the two optic cups wholly or partly fuse while a single or 

 double median lens is developed (Arch. Ent. Mech. xxiii, 1907). This 

 would seem to support the view advanced by Lewis. 



2 See above, Ch. III. 8, p. 134. 



