R. W. GERARD 2$ 



on by the molecular array m the nucleus, the gene number, kind and 

 position, so the acquired individual information must be carried somati- 

 cally, perhaps at synapses. But that experience fixation may go beyond the 

 comfortable level of ordinary neural activity is nidicated by such pheno- 

 mena as result from operative manipulation of organisms. The classical 

 experiments of Weiss and of Sperry, for example, demonstrated that the 

 discharge of impulses along motor nerves depends on the peripheral 

 comiections ; if a supernumerary sartorius muscle is implanted in the back 

 of a frog and neurotized by any nerve from the back or legs, the muscle 

 w^ill come to contract simultaneously with the normal muscle of the same 

 name. Some kind of micro-specification of the centres connected to the 

 new muscle 'teaches' them to respond to the same central activity. More 

 recently, the work of Thompson (1957, 1958) and McConnell ct al. 

 (1955, 1959) has shown that learning occurs in the essentially non-neural 

 tail of a planarian at least as well as in the ganglionated head. If an indivi- 

 dual planarian is taught a conditioned response, cut in two, and the 

 pieces allowed to regenerate, the new worm formed from the tail end 

 performs almost perfectly as soon as put to the test. 



Turning at last specifically to the nervous system, the same interaction 

 of heredity and environment is seen in full operation. Heredity gives the 

 embryonic cells which will form neurones under their normal environ- 

 ment, but become skin or lens under a different one; which will continue 

 to multiply, or biturcate, their extensions sufficiently to satisfy fully the 

 physiological field or need (Weiss, 1955); which will grow fibres in a 

 direction guided by micellar structure and by chemical concentration 

 (Cohen cr nl., 1954), to reach an appropriate end organ (as shown by 

 various regeneration experiments), or the appropriate central neurone, 

 despite operative mixing (Detwiler, 1936). But all these capacities are 

 present only in the very young embryo; at each successive stage of 

 development the potency shrinks further. Totipotent cells can later be- 

 come only neurones, distorted patterns can no longer be corrected and, in 

 general, new growth and chemical metabolism decrease m rate. Rate falls 

 off' with life time, a basic ageing process, roughly as a decaying exponen- 

 tial curve; growth and its special manifestation, learning, shrink in speed 

 and scope with advancing years until plasticity is essentially lost. Then no 

 new material trace is formed and experience is no longer fixed by the 

 individual. 



Specifically for the nervous system, the following questions are impor- 

 tant : what experience is retained ; under what conditions ; where does the 

 change occur, is it local or diffuse; what is the nature of the change, 



