120 STUDIES IN GENERAL PHYSIOLOGY 
I shall not discuss the importance of the theory of Bonnet. 
I only mention it here because it takes into consideration 
the fact that sometimes a tail may be formed instead of a 
head, which is not done in Allman’s theory of polarity. ] 
shall avoid all theoretical discussions in this paper, and con- 
fine myself to the task of showing whether and how it is 
possible to cause with certainty in an animal the growth of an 
aboral pole in the place of an oral one, and vice versa, at will. 
For the formation of an organ which in form and function 
is different from that which has been lost I shall use the 
term heteromorphosis. By the term regeneration I under- 
stand the replacement of a lost organ by one which is 
identical with that which has been lost. 
II. HETEROMORPHOSIS IN TUBULARIA MESEMBRYANTHEMUM 
A layman would be in doubt as to whether he should call 
a specimen of Tubularia mesembryanthemum a plant or an 
animal, From a much-branched system of roots (or stolons), 
which are attached to a solid substratum, arise numerous 
delicate unbranched stems, several centimeters high, which 
end in polyps that are usually red and look very much like 
flowers. These polyps take up and digest the food for the 
animal. The animals belong to the class of Hydroids and 
are found in great numbers in the Bay of Naples. 
The zodlogists have developed a very complicated ter- 
minology for the individual organs of the Hydroids, which 
may be very useful in purely descriptive morphology, but 
does not take into consideration the forms of irritability of 
the various organs. Causal morphology, which attempts to 
discover the circumstances that determine form, has to con- 
sider first of all the irritabilities of the individual organs. 
For the purposes of the physiologist it is therefore necessary 
to take these into account in describing and naming the 
various organs. 
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