HETEROMORPHOSIS 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- 
aérated 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 
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