"Polarity." 501 



GENERAL DISCUSSION OF RESULTS. 



From the data furnished by these and by previous experiments 

 we may, I think, formulate a statement in regard to the phenomena 

 of polarity in Tubularia. Several factors enter into the result: 



(i) The material of the stem is totipotent, and may produce 

 a hydranth at any level, but more quickly at an oral end of a piece 

 than at an aboral end. The quicker response at the oral end is 

 due, on my view, to the gradation of the material in the direction 

 of more to less. 



(2) The gradation is the polarity, and on this as a basis the 

 formative changes take place. Whether these formative changes 

 involve only known physical elements need not be discussed here, 

 but whatever the kind of process the gradation gives the basis for its 

 directive action — the presence of a free end calling forth the for- 

 mative changes. 



(3) The development of the aboral hydranth may appear, 

 on first thought, to contradict this idea of polarity. In reality it 

 does not do so, for the gradation is only one of a number of factors 

 that may possibly determine the result. A stronger influence of 

 another sort may call forth, in the totipotent material, the hydranth- 

 forming action. In fact, all the phenomena of axial heteromor- 

 phosis show that the polarity may be overcome; sometimes one 

 condition, sometimes another causing this result. Thus when the 

 totipotence is lost and the material can only produce one kind of 

 structure, if it produces anything, as in the tail of the tadpole, 

 the polar influence — the gradation of the material — is overcome 

 in heteromorphic regeneration from the anterior end and here a 

 tail and not a head develops. 



That even in Tubularia there is a conflict between opposing 

 factors when an aboral hydranth forms (in a piece tied at the oral 

 end) is shown by the delay in the development compared with 

 the oral development at the same level of the basal piece. 



LOCALIZATION IN EGG AND ADULT AND ITS BEARING ON DEVELOP- 

 MENT AND REGENERATION. 



A number of recent results in experimental embryology indicate 

 that the protoplasm of the egg is composed of a number of mate- 

 rials — quantitatively or qualitatively diflPerent — that go to dif- 

 ferent parts of the embryo, and become later the basis of the parts 



