THE BIOLOGICAL BASIS OF INDIVIDUALITY 



eration. The attachment of a heterotransplant to the adjoining host tissue is 

 from the beginning less complete than it would be in case of homoiotrans- 

 plantation and gradually degenerative changes occur in the transplant. 



Similar are the results if, instead of larger heterogenous transplants, small 

 pieces are grafted on defects in larger pieces. If small pieces of skin together 

 with the adjoining muscle are thus heterotransplanted into wounds in lum- 

 bricidae, they remain as a rule preserved only for a certain time; but in 

 exceptional cases the transplant may maintain itself apparently without change 

 for as long as nine months. Leypoldt used in experiments of this nature, 

 regenerating, heterogenous skin which was not yet fully developed; but in 

 most instances it was either soon discarded or it was gradually absorbed 

 through the activity of the adjoining tissue of the host. Even after the pieces 

 had healed in, in an apparently perfect condition, subsequently degenerative 

 changes took place in the transplants and led to their gradual absorption. On 

 the other hand, homoiogenous pieces of skin usually were much more readily 

 preserved for a long period of time, or even permanently. When ovaries were 

 heterotransplanted, in a small minority of cases the grafts remained in good 

 condition as long as for three months or even for one year, provided the 

 species were nearly related; but if they were farther distant, the transplants 

 were injured and were much more quickly absorbed. It is of interest that it 

 was also possible to obtain heterofertilization of the eggs which developed 

 in the heterotransplanted ovaries. The F 1 generation of such hybrids possessed 

 characteristics of both parents, but they were sterile and soon died. 



We may then conclude that heterotransplantation may succeed, although 

 usually with some difficulty, in lumbricidae as well as in the more primitive 

 hydroids and planarians, provided the species used are nearly related ; other- 

 wise the incompatibility of the organismal differentials leads to an early 

 separation or destruction of the transplant. 



Somewhat similar are the factors which are active in transplantation in the 

 oligochaeta Criodilus, as we may conclude from the experiments of Tiara. 

 In this organism certain kinds of heterotransplantation succeed, while others 

 do not. We have here, likewise, a limited degree of organ differentiation, the 

 anterior segment having the tendency to form a head, the posterior segment 

 a tail ; but as in lumbricidae, heteromorphosis may occur as a result of certain 

 mechanisms acting in such a way as to force upon a smaller piece, from a 

 distance, the formation of an organ contrary to the rudimentary differentiation 

 existing in the smaller piece. Thus a larger sized partner may gain dominance 

 over a smaller one. 



In this case also, the nervous system may perhaps determine whether or 

 not a head formation shall take place at the point of union between two 

 pieces. A head forms at the anterior cut surface if the nerve strands of the 

 partners do not unite. Under these conditions the regenerative activity of the 

 cut nerve seems to furnish the stimulus for a head formation. That in these 

 organisms a certain plasticity of organ formation still exists follows also 

 from the fact that the epidermis of the adult animal has the power to regen- 

 erate the nervous system. 



