FIELDS AND GRADIENTS 



307 



this free cut surface does not regenerate the missing (i.e. proximal) 

 regions, but produces a structure representing the parts of the Hmb 

 distal to the level of the cut, although this duplicates regions of the 

 stump. 1 The same is true of tail-fragments.^ These results show that 



Fig. 146 

 Diagram showing the morphogenetic effect of the limb-field in regeneration. In 

 Triton, an early regenerate bud from a fore-limb cut as in {a) is taken and grafted 

 on to a hind-limb stump. If (6) grafted with a portion of the original stump, it 

 produces (c) a fore-foot; if {d) grafted alone, it produces {e) a hind-foot. (Przi- 

 bram, in Handb. norm. 11. path. Physiol, xiv (i) (i), 1926.) 



the explanation given above needs modification. The field is not 

 active within its differentiated regions : the morphogenetic influence 

 is exerted in relation to the character of the diflFerentiated tissue at 

 the cut surface. 



§4 



Little is known as regards the precise time-relations of some of the 

 processes, e.g. whether the new morphogenetic gradient-field is 

 established immediately the new head is determined, or not until 

 it has reached some degree of morphological development, such as 

 the formation of a brain, or the penetration of nerves from the new 

 brain into the old tissues. The general sequence, however, is clear. 



^ References in Milojewid and Grbid, 1925. 

 ^ Milojewi6 and Burian, 1926. 



