216 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



which Moretti, Goetsch and Santos observed, is effective also in the normal 

 organism and here helps to maintain its polarity, a conception emphasized 

 especially by Child in his axial gradient theory. There are two mechanisms 

 through which such an effect could be accomplished: (1) Through contact 

 action the dominant, most differentiated part could transmit an inductive 

 effect to the adjoining posterior part and this part in sequence could exert 

 a similar inductive effect on the nearest adjoining region in the antero- 

 posterior direction ; in this way a distance action could be brought about, in 

 which the inductive effect becomes gradually weaker; or (2) substances could 

 be produced by the dominant part, from which they pass to neighboring and 

 to distant regions in gradually decreasing concentration. But there is reason 

 for assuming that such actions do not affect an indifferent material, but one 

 which, while plastic within a definite range, still possesses a certain pre- 

 differentiation which varies in different parts in fixity. 



It is possible that parts, other than the dominant part, also exert an induc- 

 tive distance effect. Thus L. V. Morgan has shown that when a head is cut 

 off in Planaria and a small piece is placed on the wound in an inverted direc- 

 tion, so that the oral pole of the graft joins the oral pole of the host, a distance 

 action is exerted by the larger piece, representing the host, which causes 

 the aboral pole of the transplant to form a head. Such an interpretation of 

 the experimental findings would apply if we assume that the head formation 

 in the transplant was not the result of unknown external factors, but was 

 caused by an integrative type of induction. 



As in the case of hydroids, so also in planarians, the mechanisms under- 

 lying the change of abnormal structures to a normal organism may lead to 

 the absorption of excess organs. In other cases the mechanism underlying 

 integration may lead to a duplication of an organism, as when two complete 

 organisms form following the production of two heads and two tails through 

 partial lengthwise incisions in the anterior and posterior parts. 



According to Goetsch, a transformation of one organ into another, a coa- 

 lescence of two organs, or a newformation of an organ, takes place by means 

 of an intermediate stage, in which, at first, under the influence of various 

 stimuli, an indifferent tissue develops, which secondarily undergoes the 

 specific differentiation. Fully differentiated organs cannot be directly trans- 

 formed. The less marked the differentiation is in a certain organism, the less 

 fargoing need be the preliminary changes, as well as the later differentiations, 

 which make possible the transformation of one organ into a different one. 

 In hydroids the regeneration following removal of a part of an organism 

 can be prevented by grafting another piece of a hydroid on the cut surface. 

 In the same way it is possible to prevent regenerative action also in Planaria. 



The facts to which we have referred indicate that the normal Planaria 

 represents an equilibrated system in which there are various mechanisms of 

 induction acting on the neighboring tissue, as well as at a distance. It is the 

 removal of these influences, as well as the direct effect of a new medium 

 surrounding the injured tissue and the altered mechanical conditions in the 



