158 T. H. MORGAN. 



that we call polarity. These material differences with their 

 correlated physiological differences lead to differences in the 

 behavior of the two ends, for, while the material exposed at any 

 cross-cut is the same at both ends, it behaves differently on 

 account of the relation of the end to neighboring regions. This 

 relation may be thought of as one of direction and this is polarity. 

 It has seemed to me possible that the relation of the parts to 

 each other might be expressed as the relative condition of 

 tension — in a physiological sense — that exists between the 

 different parts. Further analysis has led me to think that 

 behind this relation there is a more subtle one and that 

 irritability is the physiological factor that regulates the be- 

 havior of the cells in development and in regeneration. Even 

 the differentiation of the different regions must be supposed to 

 be due to their relation to neighboring regions. In fact, one 

 of the first and most obvious changes that takes place in cells 

 when removed from contact with their fellows is a loss of 

 differentiation, followed by a re-differentiation in relation to a 

 new terminal condition. Polarity therefore in the last analysis 

 stands for a graded relation resting on a condition of contractility 

 (tension) that exists between different levels. 



The Cause of the Delay in Basal Polyp-formation when 

 the Oral Polyp Develops First. 

 In my last two papers on Tubularia I have laid perhaps undue 

 emphasis on the question as to how the basal development of a 

 piece is accelerated when the oral end is tied and conversely as 

 to how the basal end is retarded when the oral end develops. 

 The problem interested me because of its apparent wider bear- 

 ings ; for it seemed possible in this case to test by suitable 

 experiments whether the result could be explained on purely 

 chemical grounds or whether a different principle was involved. 

 The attempt to find a sufficient chemical stimulus does not 

 appear to me to have been successful, for the confessedly incom- 

 plete data on which my conclusion provisionally rested has not 

 withstood a second attack. It seems to me now not improbable 

 — and more can not at present be said — that the retardation of 

 the basal development is directly owing to the formative changes 

 taking place at the oral end. 





