164 



REGENERATION 



oxygen tension. Here, too, 

 narcotics cause a reduct- 

 ion in size, which how- 

 ever can be compensated 

 by increased oxygen 

 tension (Barth, 1944). 

 In normal regeneration, 

 the role played here by 

 anaesthetics is probably 

 duplicated by inhibiting 

 substances formed in the 

 tissues (waste products 

 of metabolism). After 

 transection, regeneration 

 is started by the better 

 oxygen supply to the 

 wound area, and by the 

 disappearance of inhibit- 

 ing substances by dif- 

 fusion. 



In some cases, local 

 stimulation without re- 

 moval of material is suf- 

 ficient for a new head, 

 or, in hydroids, a new 

 hydranth, to be formed. 

 Goldsmith (1940), for in- 

 stance, caused the devel- 

 opment of supernumerary 

 heads in planarians by 

 making incisions or local 

 burns in the rostral part 

 of the body. In the earth- 

 worm a supernumerary 



head may develop laterally in the front part of the body, 

 if a wound is made there, and a cut end of the ventral nerve 

 cord is brought into the wound (Avel, 1947). Evidently 

 the nerve cord has a stimulating or activating influence, for 



Fig. 58. Correlation between the size 

 of the regenerated head and its or- 

 ganising power in Planaria. All fig- 

 ures illustrate the regeneration of a 

 caudal half of the original, cut as 

 in (a); (b) regeneration under normal 

 circumstances; (c-e) regeneration 

 under the influence of increasing con- 

 centrations of narcotics; reduction 

 in size and development of the re- 

 generated head, and corresponding 

 reduction in the size of the induced 

 pharynx (ph), and in the distance 

 between head and pharynx; (/) re- 

 generation at optimal temperature, 

 larger head and pharynx, and greater 

 distance between pharynx and head. 

 After Child. 



