268 T. H. MORGAN. 



pieces. The interpretation of this seems to be that the tendency 

 to produce hydranths, both oral and basal, is stronger near the 

 distal end and decreases basally. In short pieces the sensitive- 

 ness of the two ends to those influences that call forth the hydranth 

 is so great that both ends develop simultaneously or nearly so, 

 hence the oral end has not time to get a sufficient start over the 

 basal to stop its development. It should be noted in passing 

 that it is probable that the influence preventing basal develop- 

 ment is not only the oral development, but a direction-factor 

 present in the stem at all times. This factor we call polarity. 

 The interesting point is that this factor seems to be more capable 

 of inhibiting basal hydranth formation when an oral end is 

 developing than when such development has not yet begun. 

 The basal development, however, does not appear to delay the 

 oral process. It is acting against the polarization and its influ- 

 ence is less felt throughout the stem, as experiments by Stevens 

 and myself have shown. These considerations lead, I think, to 

 the view that the essence of our problem lies in that peculiarity 

 of the piece that we designate its polarity, and not in the absence 

 or presence of formative substances in Sach's sense. 



If our analysis is correct, we are led to look upon living ma- 

 terial as possessed of a certain formative principle that has so to 

 speak a "sense of direction." The next step will be to study 

 the nature of this principle and see what properties we are justi- 

 fied in ascribing to it ; for while it may be beyond our powers at 

 present to state precisely the nature of the directive principle, we 

 may at least be enabled to work out its manifestations. Some 

 of these manifestations become apparent in the study of the re- 

 generation of Tubularia. One of its most striking modes of ac- 

 tion is seen in the inhibition of basal-hydranth formation. Most 

 interesting is the result that its action becomes intensified by de- 

 velopmental processes going on at the oral end, as shown by 

 the fact that if the oral development is suppressed by tying 

 that end, the basal development is much accelerated. It is ac- 

 celerated in the sense that basal hydranths more often develop 

 at once than when both ends are open, but not in the sense that 

 the basal development is faster than when this end also gets as 

 early a start as the oral end. In other words, there is no speed- 



