2/4 T. H. MORGAN. 



suits of polarity on the basis of a stratification of the materials. 

 Influenced at the time by recent results in experimental embry- 

 ology that seemed to show that visible substances of different 

 kinds in the egg are really responsible for the development of 

 its parts, the same idea was applied to the problem of regenera- 

 tion, despite the fact that I had on more than one occasion re- 

 jected the hypothesis of formative stuffs, in Sach's sense, as suf- 

 ficient to account for the facts of regeneration. Yet a careful read- 

 ing of the papers here referred to will show that I still held, 

 though perhaps not always consistently, to the conception that 

 back of these differentiated materials lay the real differentiating 

 factors. 1 It now seems to me that the evidence, which at that 

 time seemed so strongly to favor the idea of the importance of 

 the grosser materials of the egg, is insufficient to establish its 

 case, and that the important factors of development are dynamic 

 properties of the bioplasm, rather than the formed products of 

 the egg, or of the differentiated products of the adult animal. 

 This statement does not mean that the visible products in the egg 

 play no role in development. The evidence still shows that they 

 may do so, but their role seems to be secondary, not primary. 



The interrelation of the parts seems to be one of the most 

 evident expressions of the fundamental formative influences. 

 Several years ago a consideration of a number of results in 

 regeneration led me to state that this relation might be expressed 

 as a sort of tension. This view has been objected to on the 

 ground that it does not appear to explain the matter any better 

 than before. In a moment of doubt and in order to give the 



l One further word of explanation. The rate of hydranth formation varies with 

 the distance of the cut end from the original hydranth. I have spoken of this differ- 

 ence in rate as explicable on the assumption of the hydranth-forming materials de- 

 creasing toward the base, i. e., away from the hydranth. It was unfortunate to have 

 used the term hydranth materials, although I made sufficiently clear in the text that 

 I did not mean to invoke the stufi-hypothesis in this connection. It is not entirely 

 clear on what the difference in rate depends ; most probably on the stem being less 

 specialized as a store-house of food substance nearer the hydranth ; probably also 

 on some difference connected with the thickness of the walls with which the speciali- 

 zation may also be connected ; possibly neither of these but some more fundamental 

 characteristic is responsible for differences in rate. In any case it is not obvious that there 

 is any connection between this difference in rate and the polarity of the piece. The 

 latter is the same for all levels the time it takes the piece to be remodelled seems to 

 be referable to something else. 



