Biology 693 



head, but one will be produced over the optic cup in the tail 

 region. 



It is largely by such influence of one part upon another that 

 the progressive increase of complexity in development comes 

 about. 



8 

 Losing a Tail to Save a Life 



In the highest groups of animals regeneration of large parts 

 cease altogether. Birds and mammals can rarely replace any- 

 thing but small losses. Sometimes the primitive power of regen- 

 eration is kept on for special reasons. Lizards, for instance, can- 

 not regenerate their limbs, but they are specially liable to be 

 caught by their tail as they disappear into their hiding-places. 

 Accordingly, we find that the vertebrae of the tail are of special 

 construction, so that they can be broken in two by a voluntary 

 muscular contraction, and the whole tail thrown off; and further- 

 more, the tail, and the tail alone of all the organs of the body, 

 can be regenerated. The animal loses its tail, but saves its life. 



This restriction of the power to regenerate is to be found 

 in individual development, as well as in racial evolution. The 

 frog, for instance, cannot regenerate lost limbs, the tadpole can. 

 Tadpoles, on the other hand, will not survive and reorganise after 

 being cut in two, but the very earliest stages of development will 

 survive this process. By tying a thread round the developing egg 

 of a newt so as to constrict it into two separate parts, the egg 

 may be made to give rise to two perfectly normal newts instead 

 of the single one that it would normally form. This has a bear- 

 ing upon human problems. As everyone knows, there exists two 

 kinds of twins those which are not more alike than ordinary 

 brothers and sisters, and may be of either sex, and those (the 

 so-called identical twins) whose resemblance is so striking as to 

 lead to confusion and whose sex is always the same. This latter 

 kind of twin is produced when the embryo in its very earliest 

 stages of growth is, by some accident or other, separated into two 



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