HETEEOMORPHOSIS 123 



ment of a head at both ends of the fragment of a stem, but 

 also to prevent the formation of a head at will, by simply 

 putting this pole into the sand. When both poles are put in 

 the sand, no head is formed at either end. If one of the 

 poles which has been in the sand for some time, and on 

 which the formation of a head has been prevented in this 

 way, is pulled out of the sand so that it is again surrounded 

 on all sides by water, a head may form at this end. If the 

 animal is covered only by an exceedingly thin layer of sand, 

 a polyp will still be formed which makes its appearance 

 between the grains of sand, much as the stem of a ger- 

 minating seed may grow through a thin layer of earth. 



One of the poles of a piece of a stem was pushed between 

 two slides laid upon each other and held together by thin 

 rubber bands. Needles were placed between the two slides, 

 and one end of the stem of the Tubularian was laid in the 

 wedge-shaped space thus formed. In this way the end was 

 subjected to slight pressure. No polyp was formed at the 

 end subjected to this slight pressure, no matter how long I 

 waited; while at the other end, which was not pressed upon 

 and was surrounded by water, a polyp was formed in the 

 usual time. When the piece was removed from between the 

 slides, a new polyp frequently developed at the end that had 

 been subjected to the pressure. That light is not necessary 

 to the formation of a polyp was proved by the fact that 

 pieces of Tubularian stem will grow new polyps in a dark- 

 ened vessel. The experiments described were made in well- 

 aerated aquaria. 



4. When the polyps and the roots are cut off from long 

 stems of Tubularia mesembryanthemum, it is found that the 

 new polyps are always formed one, two, or three days earlier 

 at the oral than at the aboral end. I believe that the cause 

 of this phenomenon, which may be considered as an intima- 

 tion of "polarity," lies in the fact that when long pieces are 



