124 STUDIES IN GENERAL PHYSIOLOGY 
cut from the stem, the lumen at the cut oral end is usually 
wider than that at the aboral end; for when I cut from the 
middle of the stem shorter pieces, which showed no differ- 
ence in the diameter of the lumina, a polyp often formed 
earlier at the aboral end than at the oral. 
5. The size of the newly formed polyp also depends to a 
certain extent upon the diameter of the stem at the cut end. 
When the diameter was very small, the polyp was also very 
small; when the diameter was large, the polyp was also 
larger. 
6. It might still be imagined that, besides the mechani- 
cal factors thus far considered, a physiological factor might 
also play a réle. It might be thought that the substance of 
which the polyp is formed is present in a larger amount at 
the oral than at the aboral pole. To test this point I chose 
a large number of very long Tubularian stems that had been 
cut off close to the roots, and at the cut ends of which polyps 
had been grown. I bisected these stems transversely, and 
kept the oral and aboral halves in separate beakers. If the 
substance required for the formation of the polyps were 
unequally distributed in the stem, then the one series of 
fragments should have formed polyps sooner than the other 
series. This was never the case; but—as was again noted 
—every fragment formed a polyp sooner at its oral than at 
its aboral end, even though the difference in time often 
amounted to only one half-day or less. 
7. While I have always succeeded—with suitable mate- 
rial, and with the experiment under the proper external con- 
ditions—in making a head grow at the aboral end of the 
stem, I have thus far not yet succeeded in making a root 
grow at the oral end of a stem. When I cut off the stems 
close to the substratum to which the roots were attached 
and brought the aboral ends in contact with the walls of the 
aquarium, the end, when it grew at all, attached itself to the 
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