COELENTERATES AND PLANARIANS 205 



The existence of a rudimentary differentiation in coelenterates is well 

 exemplified in the experiment of Burt, who showed that rings taken from 

 the anterior pole have, after transplantation, a greater tendency to form heads 

 than have foot pieces, which latter have a greater tendency to form a foot, 

 although in exceptional cases here, also, a rudimentary head formation with 

 tentacles can be induced. This predifferentiation can be overcome under cer- 

 tain conditions : ( 1 ) By the application of various external factors. In this 

 way Jacques Loeb first produced heteromorphosis in Tubularia. (2) Also 

 by the action of factors present within the organism; namely, when certain 

 differentiated areas, in some cases strange organs, introduced by means of 

 transplantation, extend their influence to other areas. Here we must again 

 distinguish two sets of factors: (a) The action of contact substances, or — 

 to use a more general term — contact mechanisms. Through contact with a 

 differentiated transplant, as, for instance, with the head of a Hydra, the 

 anterior pole or the foot region of the host can be induced to form, at or near 

 the place of contact, an organ corresponding to the transplanted head. In this 

 case we must assume that contact mechanisms (contact substances) induce 

 a heteromorphosis, inasmuch as a new formation takes place at a point where 

 normally another part of the organism would have developed. Thus, a head 

 may be induced to form in a place where normally a foot had been, or it may 

 form in the middle zone. The resistance to such a head formation is greater 

 at the aboral part than at the oral part, owing to the predifferentiation of the 

 host organism. We have, here, to deal with an organizer action comparable 

 to that which plays so important a role during embryonal development. It is 

 especially transplantation of a regenerating hydranth, but also of other kinds 

 of tissues, such as parts of Hydra buds and peristome, which in a specific 

 manner induces the formation of hydranths in Hydra viridis (E. N. Browne, 

 Goldsmith). In addition to these factors which thus lead to the formation of 

 supplementary organs, there may be active another factor, which consists 

 in the tendency of an organ to inhibit the formation of an organ of the same 

 kind, especially in its close proximity. From such a near point this inhibiting 

 influence may be transmitted to more distant parts, but apparently with 

 decreasing intensity, (b) Factors of a regenerative or restitutive character. 

 These may tend to supplement a part of an organism which has been separated 

 from the rest, in such a way that the formation of a whole organism results. 

 In this latter interaction a more differentiated tissue again is generally more 

 potent in determining what organs shall be produced, than one less differen- 

 tiated — it is the dominating, directing constituent of the organism. Moreover, 

 a larger part usually prevails over a smaller part, other conditions being 

 equal. While the organizer action in coelenterates mentioned above may lead 

 to the reproduction of the same organs as are present in the organism, the 

 regenerative tendency on the other hand leads to the newformation of sup- 

 plementary, therefore of different, organs or areas. There is another, more 

 definite difference between the mode of action of such an organizer and of 

 the integrative restitutive or regenerative factors. The former acts, as stated, 

 presumably through contact substances or contact mechanisms, while the 



