The Phenomena of Polarity in Tuhidaria. 591 



the development of a polyp at the aboral end. The polyp when 

 first formed is small, and can, in fact, be seen to enlarge for several 

 days after it has emerged. 



I have assumed that the stem of Tubularia is not homogeneous, 

 but that from the hydranth to the base there is a graded difference 

 and this gives the order or stratification of the material. The 

 stimulus of the water acting on the free end arouses the form- 

 ative changes which act in the direction of this existing material 

 order. How does such a view diflPer from the old assumption of 

 a "polarity" in the material .? On my view there is no such 

 directive force residing in the material as the term polarity 

 suggests, but the polarity is only a name for the gradation of 

 the material and on this as a basis the formative changes are 

 carried out. 



Conversely a similar gradation from the stolon to the hydranth 

 must exist in respect to the tendency to produce a stolon, hence 

 the latter appears on the aboral end; but in the case of Tubularia 

 we find that the action of water on a free end, whichever this 

 may be, has a stronger tendency to call forth a hydranth than a 

 stolon in some species. Hence the conflict of influences that 

 probably goes on at this end. If, however, the aboral end is 

 brought in contact with a solid body a stolon develops promptly, 

 while the oral end, if brought into contact with a solid, fails 

 to produce a stolon, but develops a hydranth. 



In conclusion, I think, that the Sachs-Bonnet hypothesis of the 

 migration of formative stuff^s is not needed to explain the results 

 in Tubularia, and furthermore that there are no observed facts 

 or experiments that make this view probable. I have no wish to 

 deny that substances having a formative influence on growth 

 may exist, but there is no evidence in favor of the view that in 

 multicellar forms, such substances migrate in definite directions 

 and thus produce the "polarity." If development begins in a 

 region, there may be a movement of the materials that are being 

 used up toward the developing part, but to fail to distinguish 

 between this point of view and the former is to confuse cause and 

 eflPect. The two points of view are diametrically opposite, and 

 no good end can be reached by ignoring this fact. 



