210 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



While thus the difference in race- and speciesdifferentials in the pieces 

 joined together may lead to antagonistic actions between the tissue consti- 

 tuents of the different grafts, yet to a certain extent it is possible for the 

 tissues in such buds and chimaerae to live and grow side by side without the 

 manifestation of a hostile reaction. Somewhat comparable results can be ob- 

 tained, as we shall see later, in the transplantation of regenerative buds of 

 extremities in amphibia, in the ingrowing of sidelines from one partner into 

 the other in heterotransplanted amphiban larvae, or, as we have mentioned 

 already, in the ingrowth of a nerve from one partner into the other in para- 

 biosis in rats. In all of these conditions there is a lack of manifest reaction on 

 the part of tissues which are in close contact with each other, although they 

 differ in their organismal differentials. In such cases we have to deal either 

 with ontogenetically or phylogenetically very primitive forms, or with re- 

 generating tissue which does not yet possess the fully developed organismal 

 differentials, or at least the mechanism of reaction against such differentials. 

 In the case of parabiosis in rats we may have to deal with relatively slight 

 differences in organismal differentials. 



In accordance with the experiments mentioned above, Mutz found that 

 pieces of Hydra and Pelmatohydra can be joined together in the long axis of 

 the body, the different constituents retaining the character of their own 

 species. However, the growing together takes place with much greater diffi- 

 culty than in homoiotransplantations and for a long time the place of union 

 remains visible; but in the end a uniform, apparently normal Hydra, though 

 in reality representing a chimaera, may result from this transplantation. On 

 the other hand, the green Chloro hydra cannot be joined to the brown Pelma- 

 tohydra, to Hydra vulgaris or Hydra attenuata, separation taking place within 

 eight days. It seems that in this case the presence of algae in Chlorohydra 

 intensifies the difficulties of heterotransplantation. While in the case of 

 homoiotransplantation algae do not interfere seriously with the result, this is 

 not so if distinct races or species of Hydra are combined; then, the presence 

 of Algae increases the incompatibility between the partners, as the experi- 

 ments of Goetsch have shown. But even if the number of algae is approximate- 

 ly the same in different partners, there still remains noticeable the difference 

 in race or species constitution. If the head of Hydra vulgaris or attenuata is 

 transplanted to Pelmatohydra, the union between transplant and host is only 

 a temporary one, lasting usually from three to five days, or, at most, two 

 weeks. In this instance, the transplant is not able to act as readily as an or- 

 ganizer, inducing a head formation in the host, as it would have been if the 

 transplant and host had been homoiogenous. There forms, instead, at first a 

 bridge of tissue, growing from the host in the direction towards the transplant. 

 This bridge represents a somewhat indifferent kind of tissue in which tenta- 

 cles are lacking; but after separation of the transplanted head and host has 

 taken place, it may develop in some cases into a small head, while in others it 

 is drawn into the host and absorbed. The organizer action is therefore inter- 

 fered with in such a heterotransplantation. However, when reciprocal or- 

 ganismal transplantations of pieces of Pelmatohydra and Hydra attenuata 

 are made, the two pieces may remain united long enough to make possible the 



