206 STUDIES IN GENERAL PHYSIOLOGY 



cdef is cut from the body of an Actinian, tentacles are 

 formed upon only one of the four cut edges, namely, upon 

 the oral surface ef. From this and similar experiments 

 which may be read in the original it follows that the place 

 where the new tentacles are formed in a fragment of Cerian- 

 thus is determined by the orientation which 

 the fragment had in the intact animal; just as 

 in every fragment of a broken magnet the 

 position of the poles is determined by the 

 orientation of the piece in the unbroken 

 magnet. From this analogy which, how- 

 ever, goes no further the term "polarity" 

 has recently been applied to the organization of 

 such animals. According to this idea, Cerian- 

 thus would be a typical "polarized animal." 

 Now, Cerianthus is not a satisfactory sub- 



FIG. 52 



ject for investigation of the causes of "polar- 

 ity ;" Tubularia mesembryanthemum, a Hydroid, adapts itself 

 much better to these experiments. 1 Tubularia ends in a root 

 at its aboral end, and in a polyp at its oral end. If a piece 

 is cut from the stem, and both cut ends are surrounded by 

 water, a heteromorphosis results, as I have shown in a former 

 paper. A polyp arises from both cut ends, and a bioral animal 

 is obtained. Yet the formation of polyps is different at the 

 two ends in one particular: it is always more rapid at the 

 oral than at the aboral pole. In so far as this fact renders 

 it possible to determine which was the oral and which the 

 aboral end in the uninjured stem, it can still be considered 

 as an expression of polarity. The following experiments 

 deal with the internal conditions which determine the differ- 

 ence in time between the formation of the polyps at the oral 

 and at the aboral ends in Tubularia. 



The starting-point of these experiments is J. Sachs's 



1 See Part I, p. 115. 



