220 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



than in planarians and the relative importance of inductive factors, acting 

 by means of distance substances given off by transplanted pieces, is dimin- 

 ished as compared with the more primitive organisms, although it is not yet 

 entirely lost. Likewise, the action of external environmental factors on organ 

 formation is not evident in lumbricidae in the sense in which it exists in the 

 case of hydroids. Korschelt and Mutscheller have shown that the ability to 

 form a head is limited to the most anterior part of the body, and that the 

 farther back the segments are, the greater is their tendency to form a tail. 

 The gradient in organization in Lumbricus is thus quite definite. If one 

 transplants to the anterior pole of an animal, whose head has been cut off, 

 a posterior part (tail) of a worm in the inverse direction and then cuts off 

 the end segments of the transplanted tail, the inductive action of the large 

 posterior piece, which would tend to call forth the production of a hetero- 

 morphic head at the end of the graft, cannot overcome the strong organ- 

 specialization of the tail segments which have the inherent tendency to produce 

 a tail. But if a still larger number of the posterior segments of the tail piece 

 are removed before a remaining piece of the tail is grafted inversely on the 

 anterior cut surface of the host, then the inductive, integrative action of the 

 larger partner can overcome the less specialized organ differentials which 

 exist in the graft, at a somewhat greater distance from the posterior end of 

 the animal, and formation of a head may take place at the free cut surface. 

 There exists, thus, a competitive struggle between the inductive action of 

 substances or mechanisms, which act from a distance and which may tend to 

 produce a heteromorphosis, and the fixity of the organ differentials in the 

 transplant, which, when unopposed, would lead to the reproduction of a tail 

 organ. In this struggle the larger partner has an advantage over the smaller, 

 a condition applying similarly in hydroids and in planarians. A corresponding 

 experiment which L. V. Morgan carried out in Planaria, leading to the 

 heteromorphic development of a head at an aboral cut surface grafted in- 

 versely on the anterior cut surface of a larger piece, succeeded more readily, 

 because in planarians the organ differentials are not yet fixed to the same 

 degree as in lumbricidae. The more pronounced differentiation of organs in 

 these latter animals diminishes the plasticity of the organism and the ready 

 transformation of polar organs, as well as the importance of integrative 

 induction. 



More recent experiments of Julian Huxley and Gross with the polychaete 

 worm, Sabella, indicate that the making of a wound as such may exert a 

 stimulus which acts not only locally in an area adjoining the wound, but 

 which may also act at a distance and influence the character of the structural 

 changes which shall take place. Thus the cutting off of a regenerated head 

 may not only influence a transformation of abdominal segments into thoracic 

 segments near the head pole of the animal, but it may also cause, under 

 certain conditions, a further transformation of regenerating abdominal seg- 

 ments situated at the tail pole of the animal into thoracic segments. Also, 

 small lateral wounds may influence the changes which take place in adjoining, 

 and even in more distantly situated segments ; these changes may consist either 



