280 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



extremity, but at the sides where the grafted limb did not fully cover the 

 remnant, new extremities grew out. In this instance, therefore, the inhibition 

 exerted by the graft was strictly limited to the place of contact. If, instead of 

 grafting another bud onto the exposed surface of the regenerative bud, it was 

 completely covered with a piece of skin or with a layer of muscle and skin, 

 the regeneration was entirely prevented. Possibly here mechanical factors 

 also played a role, as they apparently did in the experiments of Schaxel. 



Another interesting example of the antagonistic action between transplant 

 and host, and the latter's tendency to grow or regenerate, is the inhibiting 

 effect shown, in various degrees, by the morphogenic gill field on the de- 

 velopment of transplanted limbs. In the presumptive gill region ectoderm and 

 mesoderm have the tendency to produce gill structures, a tendency which 

 is graded in intensity in different areas (Ekman, Detwiler) ; this inhibiting 

 effect is evidently of a specific nature and it leads to a struggle between the 

 transplant and the host tissues, which mutually antagonize each other in the 

 realization of their morphogenic tendencies. These effects consist presumably 

 in contact actions. Very fine differentiations which take place during em- 

 bryonal development in this area are made manifest by means of transplan- 

 tation, and they determine the character of the contact actions. Thus, in 

 general, the nearer the ectoderm used for transplantation is situated to the gill 

 region in the donor, the more it is forced to conform to the influences exerted 

 by the underlying tissues in this area, which tend to convert the transplant 

 into gill structures and at the same time to suppress limb formation. 



The specificity of the factors which are active in the inhibition of regenera- 

 tion is, perhaps, most convincingly demonstrated in some experiments of 

 Harrison, which concern the production of heteromorphic tails in larvae of 

 Rana. Two anterior parts of these larvae were united, each with the aboral 

 pole of the other. If a piece was cut off from one of the combined anterior 

 parts a tail regenerated, in which the medulla of the head part, which had 

 been left intact — the new host — and that of the second partner — the graft — 

 and its regenerate were united, but in which the chordae were not united. 

 Under these conditions the free end of the chorda of the dominating host 

 stimulated regeneration of an additional tail, which possessed chorda but in 

 which the medulla was lacking. Evidently the surface of the medulla in the 

 graft, which fitted the surface of the medulla in the host and regenerate, 

 prevented a new regenerative outgrowth of the medulla of the host into the 

 additional tail. On the other hand, the surface of the chorda, not being in- 

 hibited by contact with a suitable surface of chorda tissue, regenerated and 

 gave rise to the newformation of a tail. In this case, also, the inhibition must 

 have been of a specific character ; medulla inhibited medulla, but the chorda, 

 not being specifically inhibited by an adequate surface of chorda, grew out 

 and gave rise to regeneration. Here we can therefore exclude simple mechani- 

 cal factors as inhibitors of regeneration. 



Whether there will be compatibility or lack of compatibility between host 

 and transplant depends also upon the degree of self-differentiation which has 

 been reached in the development of both host and transplanted tissues. As 



