170 



REGENERATION 



gradient-field can be 

 brought into contact by 

 transplantation. In this 

 case, a strong prolifera- 

 tion of tissue occurs at 

 the boundary (Okada and 

 Sugino). The greater 

 the discontinuity in the 

 gradient, the more marked 

 is this proliferation. This 

 was beautifully demon- 

 strated in an experiment 

 by Schewtschenko (1937), 

 who grafted heads of 

 planarians into different 

 regions of the hosts. Im- 

 planted caudally, the 

 heads caused a very 

 strong outgrowth, but in 

 grafts in more rostral 

 areas the effect became 

 gradually weaker. Trans- 

 plantation of caudal parts had exactly the opposite effect: 

 strong proliferation in rostral areas, much weaker proliferation 

 more caudally (Fig. 62). 



One of the effects of the "dominant" area (Child) on the 

 subordinate parts of the field is of an inhibitory nature. Miller 

 (1938) transplanted the head of a planarian into the trunk 

 area, and then transected the host just rostral to the graft. 

 No head was then regenerated at the front edge, because the 

 graft repressed such regeneration. A head grafted into the 

 trunk sometimes moves in a forward direction because (1) 

 strong proliferation takes place at the caudal side of the graft, 

 and (2) the tissues in front of the graft are resorbed. This 

 movement goes on until the head has found its way to the 

 rostral end. 



The apical "organiser", therefore, has a twofold influence. 

 It prevents the formation of new apical parts, and it forces 



Fig. 62. The effect of discontinuities 

 in the gradient in Planaria. (a) the 

 growth caused by grafted heads is 

 the more marked when the head is 

 implanted more caudally; (b) on the 

 other hand, transplanted hind ends 

 produce strongest growth in the 

 rostral parts of the host. After 

 Schewtschenko. 



