SUBSTRATUM AND MORPHOGENIC SUBSTANCES 449 



was not able to restitute lost parts. Weiss concludes therefore that the leg has 

 lost the ability to act as an organizer. However, the interpretation of these 

 experiments suffers from the difficulty that there is some uncertainty as to 

 whether, in Triton, a real transformation of the grafted tail into leg took 

 place, or whether, instead, a regeneration may not have proceeded from 

 the remaining stump of the limb. 



On the other hand, Guyenot had shown previously that if, following 

 metamorphosis, an extremity of Bufo vulgaris, which no longer possesses the 

 ability to regenerate lost parts, is grafted to a larva of Salamandra maculosa, 

 which latter is able to regenerate extremities, the transplant heals in but has 

 not gained thereby the power to regenerate lost parts when a portion of the 

 transplanted limb is amputated. This indicates that the lack of regeneration 

 depends upon conditions inherent in the transplanted Bufo tissue, and that 

 the presence in Salamander of substances able to stimulate the growth of a 

 leg, if such substances should exist, is of no avail. In these instances we have 

 to deal with examples of organ rather than of organismal specificity. 



Axolotl does not possess a balancer, while Triton does have this organ. 

 But notwithstanding the lack of a balancer in Axolotl, the medullary plate of 

 this species contains an inductor substance able to cause the formation of this 

 structure in the kind of tissue which has the potentiality to produce this organ. 

 Therefore, if the anterior portion of the medullary plate is transplanted from 

 Axolotl to a later gastrula, or to an early neurula stage of Triton, the trans- 

 plant may induce in the host epidermis the formation of a balancer, while 

 this effect is lacking if the medullary plate is in contact with the Amblystoma 

 epidermis. The reason then why Amblystoma does not possess a balancer is 

 not due to the lack of the proper stimulus, but to the inability of the tissue to 

 respond to such a stimulus in an adequate manner. 



The analogy between this condition and the findings of Schotte, to which 

 we have referred in a preceding chapter, is evident. After transplantation 

 of Rana tissue to Triton the oral region of the host supplied an organizer 

 substance, which induced the formation of mouth organs in the transplant ; but 

 the potentiality of the transplanted tissue itself determined the specific kind of 

 mouth organs which actually developed under the influence of the inductor 

 tissue. In both of these cases we have to deal with examples of organismal 

 specificities inherent in the reacting tissues, whereas the organizer substance 

 does not manifest an organismal specificity. 



There exist, however, conditions in which the lack of reaction is due not 

 to the specificity of the recipient tissue but to the lack of a hormone. Thus 

 Wigglesworth, Piepho, and others, have demonstrated that in the larvae of 

 insects changes leading to pupation are induced by a hormone which is local- 

 ized in certain parts of the brain — or rather in the ring gland situated be- 

 tween the hemispheres of the larval brain (Hadorn and Bodenstein) — and 

 which may circulate also in the bodyfluids. Now, it can be shown that changes 

 characteristic of the pupa may be induced even in the skin of the imago if the 

 larval pupation hormone is supplied. This hormone does not possess finer 

 organismal differentials and presumably lacks them altogether; therefore it 



