5o6 T. H. Morgan, 



are, in part, the outcome of this gradation; in part also of the kind 

 of substances in a given region and also of a formative action 

 using the preceding conditions as a basis, as v^ell as taking into 

 account the amount of material present. The formative influence 

 acting in a centripetal direction always gives precedence, as it 

 were, to the terminal organs. In regard to the specification of the 

 cytoplasm, as the basis of regeneration, versus the assumed toti- 

 potence of the nuclei, we have at present a choice of three views, 

 no one of which can be said to be satisfactorily established: 

 (i) The cytoplasm alone furnishes the basis for the action of 

 the formative changes without regard to whether the nuclei are 

 storehouses of hereditary qualities or whether they have to do 

 only with the feeding (and respiration ?) of the cell — not directly 

 with its growth and specification. (2) The nuclei are reserve 

 storehouses of all of the hereditary elements and may be called 

 upon to supply whatever is needed for the formation of the new 

 part, the cytoplasm returning to an "embryonic condition" to 

 be worked over under nuclear control. This view is the logical 

 outcome, it seems to me, of the current view in regard to the 

 relation of nucleus and cytoplasm. I have tried to show by a 

 brief examination of the evidence on which this view rests that 

 it is not established beyond doubt. (3) Both nuclei and cytoplasm 

 may be progressively specialized, hence it may not be profitable 

 to make any distinction between the part they play in regeneration. 

 The gradation of the material — the polarity — is, on this view, 

 expressed as much by one as by the other, and by both alike. 



