REGENERATION AND TISSUE EQUILIBRIUM 279 



which they produce. While ectoderm taken from the area covering the devel- 

 oping extremities may favor the regenerative growth of embryonal buds, or 

 at least does not inhibit it, ectoderm taken from the head region does inhibit 

 it, but only if this ectoderm has reached a certain stage of development. 

 Similarly, Mangold observed that the epidermis of Axolotl, which does not 

 possess the ability to produce a balancer, may exert an inhibiting effect on skin 

 which otherwise would be able to produce this organ. We would have, then, 

 in this case, to deal with specific effects of the transplanted epidermis on the 

 regenerative process and not with non-specific pressure effects; but while 

 these relations between epidermis and underlying cutis are specific and not 

 purely mechanical in their action, they are specific in a particular way and 

 not exactly identical with the effects observed by Weiss and others. There 

 are involved, here, tissue equilibria of a special nature. According to the 

 observations of Weiss, a transplant inhibits even the onset of regeneration 

 if the two surfaces joining transplant and host are mutually perfectly ade- 

 quate. Under these circumstances a very rapid union between the two pieces 

 takes place. We may assume that the transplant brings about the same condi- 

 tion at the point of junction which would prevent regenerative growth proc- 

 esses in this area in the normal intact organism ; in the latter, the normal 

 neighboring tissue exerts presumably the same kind of inhibiting contact 

 effects as does the grafted, strange tissue under experimental conditions. 

 Inasmuch as in many of these experiments there are successful homoiotrans- 

 plantations, we may furthermore conclude that even homoiogenous differen- 

 tials make possible these normal interactions of equilibrating contact mecha- 

 nisms in amphibia, and also that a very brief interruption of the contact action, 

 such as occurs during the excision of a piece of tissue and the grafting of 

 another piece in its place, is not sufficient to initiate growth processes. But 

 if these contact actions are not completely adequate, graded differences in 

 incompatibility may exist in different cases between transplant and host and 

 then it is possible for the regenerative outgrowth of the host tissue to take 

 place even at a time when the union with the transplant has become already 

 so firm that this outgrowth is unable to induce the casting off of the trans- 

 plant ; instead, a struggle may develop between the two tissues and the trans- 

 plant may be pushed sidewise by the regenerating host tissue, so that in the 

 end it forms an appendage to the regenerated extremity and a double forma- 

 tion is produced. In this case the mutual antagonism between host and trans- 

 plant manifests itself in an inhibition of growth of the transplant; but the 

 more subtle mechanisms of attack by means of specialized cells of the host, 

 which we can observe in mammalian transplantation, are, as yet, apparently 

 lacking in these more primitive organisms. 



Similarly in the experiments of Milojevich, who used Triton extremities 

 directly after metamorphosis, the surface of an extremity was partly, but not 

 entirely, inhibited from growing out by grafting onto it the regenerative bud 

 of another Triton limb. If the latter was at such an early stage of development 

 that the tissue differentials had not yet fully formed, then the outgrowing 

 part of the remnant of the host bud and the grafted bud united to form one 



