HETEROMORPHOSIS 121 
I distinguish in Tubularia, according to the differences 
in irritability, between the stems and the root. By the root 
is understood that part of the Tubularian which is endowed 
with a special contact-irritability (stereotropism), by virtue 
of which it attaches itself to solid bodies and keeps the 
animal in a fixed position. By the stem is understood that 
part of the animal which bears the sexual elements and the 
polyps, and which is endowed with the opposite irritability, 
in consequence of which it grows away from the substratum 
to which the animal is attached. This simple terminology, 
which is based upon the irritability of the organs, will 
suffice for our purposes. Of the entire animal only the 
polyps can move spontaneously; the stem is immovable. If 
we cut a piece out of a stem, we must discriminate between 
its oral and aboral ends, according to the orientation of the 
piece in the original uninjured animal. The oral end is 
that which was originally directed toward the polyps, the 
aboral end, that which was directed toward the root. I 
shall now describe the main experiments individually. 
1. I cut off the roots and polyps of a series of stems, 
and put these mutilated stems with their aboral ends ver- 
tically into the sand sufficiently deep to keep them in a ver- 
tical position. At the free oral ends, which were surrounded 
on all sides by sea-water, new polyps were formed in a short 
time—at the proper temperature and with favorable speci- 
mens within two days. These corresponded in form with 
the old polyps. No growth took place at the ends which 
were buried in the sand, no matter how long the observations 
were carried on (in some instances for several months). 
When I put stems with their oral ends in the sand, a 
polyp was formed at the free, aboral pole. In favorable 
cases this was formed in a few days. Neither a polyp nora 
root was formed at the oral end, which had been covered by 
sand, no matter how long I continued my observations. 
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