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. I 

 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 hcleromorphosis. By the term reyenrrafion 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 mesembryanthemtim 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 zoologists 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. 



