692 The Outline of Science 



If we alter the inner machinery of the animal, or if we 

 alter the outer environment, the balance may not be the same as 

 before, and we shall get abnormal results. For instance, below 

 certain temperatures, a half flat-worm will not grow a new head 

 at all, while at high temperature it will grow one which is larger 

 than the normal. Here we have altered the outer world. The 

 most remarkable example of an altered result due to an altera- 

 tion in the animal is to be found in various prawns and shrimps. 

 These have their eyes mounted on stalks, and the part of the 

 brain connected with sight is to be found near the top of the 

 stalk. If the eye alone is cut off, a perfect new one is regener- 

 ated, but if the eye is cut off together with the part of the brain 

 in the stalk, what is regenerated is not an eye, but an organ 

 identical with the first of the animal's two feelers or antennae. 

 This happens only when the regenerating nerve makes a con- 

 nection with the rest of the brain, so that here it is clear that the 

 different parts of the brain have an influence in determining what 

 the regenerated part shall be like. 



A somewhat similar example, showing the influence which 

 one part may have over another, is seen in the formation of the 

 lens of the eye in young newts and salamanders. The eye has 

 a compound origin in development. The sensitive part or retina 

 is derived from a cup-like outgrowth arising from the brain the 

 optic cup; the lens is formed as a thickening in the skin just over 

 the cup. Various experiments have proved that the formation 

 of the lens at the exact place in which it will be useful is due 

 to some chemical influence ( of the same nature, no doubt, as that 

 exerted by the ductless glands see p. 697) exerted upon the 

 skin by the developing optic cup. If the skin is removed from 

 the side of the head over the cup, and a piece from some other 

 part of the body, or even from some other animal, is grafted on 

 in its place, the new grafted piece will produce a lens. But if 

 the optic cup is taken out and grafted under the skin elsewhere, 

 for example, near the tail, no lens will be formed in the 



