304 



FIELDS AND GRADIENTS 



Amphibia. It has always been difficuh to connect this with any 

 purely chemical specificity of the regenerating tissues at one level 

 as against another level of the limb, and recent work has made such 

 a view wholly untenable. For one thing we have the fact already 

 referred to (p. 271) that the regenerated material is at first wholly 

 undifferentiated, and is only later determined in relation to the 

 substrate on which it grows. This is not conclusive, for it merely 

 proves that the old tissues do not impart any chemical specificity 

 they might possess to the material just proliferated ; the later deter- 

 mination might be due to chemical influences specific to the level 



Fig. 144 

 Diagram to show the independence of regenerated tissues, a, Triton with normal 

 fore-limb skeleton, b, The humerus is removed, and the fore-arm and hand re- 

 moved, c, The regenerated fore-arm and hand contains the normal complement 

 of skeletal elements. (Przibram, in Handh. norm. u. path. Physiol, xiv (i) (i), 1926.) 



of the cut. However, it has now been shown that total absence of 

 one kind of tissue, or the substitution of one kind of tissue by another 

 in the regenerating base of the limb, does not interfere with normal 

 regeneration. If the skeleton be removed from the upper arm or 

 thigh region of a Urodele limb, and the limb later cut across in this 

 region, the distal regenerated portion possesses a normal skeleton, 

 whereas no regeneration of the missing parts occurs in the stump. ^ 

 Similarly, if the skin is removed from a limb, an envelope of lung 

 tissue grafted on, and the limb cut across after healing has occurred, 

 the regenerated portion is found to possess normal epidermis, in 

 spite of the absence of such tissue in the stump.^ 



^ Weiss, 1925; Bischler, 1926. ^ Weiss, 1927 a. 



