HIGHER INVERTEBRATES AND AMPHIBIA 221 



in a loss of certain structures, followed by the formation of new structures, 

 or, in some instances, in the direct transformation of organs, without the 

 previous loss of other structures. These observations point to the existence 

 of a somewhat furthergoing plasticity in the structure of annelids, which 

 permits the transformation of regenerating abdominal segments into segments 

 with the character of thoracic segments. On the other hand, experimentally 

 produced duplications of considerable size may persist unchanged under 

 conditions in which, in the more primitive and plastic organisms, various 

 regulative mechanisms would have eventuated in the formation of normal 

 individuals. As far as the organismal differentials are concerned, homoioge- 

 nous combinations are possible in lumbricidae, without leading to disharmonies 

 which, as we have seen, take place in parabiotic partners in mammals, owing 

 to the greater refinement of organismal differentials in the latter. 



We see, then, that in general the differentiation and fixity of the organism 

 is much more advanced in lumbricidae than in hydroids and planarians ; 

 correspondingly, the inductive distant action has decreased in effectiveness 

 in the former, and it is likewise due to their relative fixity in organization 

 that an organizer action, in which a differentiated part grafted on a host 

 induces here the development of its own kind of an organ, seems not to have 

 been observed in this class of animals. Such an organizer would probably be 

 unable to act effectively with this less plastic material. On the other hand, 

 there is reason for assuming that the second kind of inductive action leading 

 to integrative regeneration is still, though to a much diminished extent, potent 

 even in lumbricidae. 



Heterotransplantation succeeds in lumbricidae with much greater difficulty 

 than homoiotransplantation. In the large majority of cases heterogenous 

 pieces remain united only for a few days, then separate or degenerate. In 

 other cases there may be a better union by means of scar tissue covered by 

 epithelium ; secondarily, muscle, nerves and vessels may grow through it and 

 into the heterogenous tissue. Thus also in heterotransplantation blood vessels 

 as well as other organs may unite with the corresponding organs of the partner 

 and a common circulation be established. But even under these conditions 

 while there is, at first, an apparently perfect union of the two heterogenous 

 pieces, a separation may take place subsequently and as late as after five 

 weeks. As a result of changes occurring at or near the place of union, the 

 latter becomes looser and a mere mechanical pull may readily separate the 

 two partners. However, in one instance Korschelt succeeded in keeping a 

 combination of Lumbricus rubellus and Allophora terrestris alive for a period 

 of eight to nine months. In pieces thus temporarily united, in which a smaller 

 heterogenous piece has been grafted in an inverse direction on a larger host 

 whose head has been cut off, the host may induce the beginning formation 

 of a heteromorphic head in the smaller anteriorly situated graft; but usually 

 at the point of union a new head develops and then the pieces separate. The 

 heterogenous contact substances, or, more generally expressed, contact 

 mechanisms, which are active at the cut surfaces do not keep the adjoining 

 parts in an equilibrated, quiescent state and can not therefore prevent regen- 



