3^ 



BIOLOGICAL LECTURES. 



physiologist the question arises: What are the circumstances 

 which determine that only one kind of organs originate at 

 certain places in the body ? I conceived that the answer to 

 this question might be obtained by finding out 

 whether or not it was possible to make any 

 desired organ of an animal grow at any desired 

 place. In case of success, the question to be 

 decided was whether the same circumstances by 

 which we can change the arrangement of organs 

 experimentally determine also the arrangement 

 of organs in the natural development. 



If we cut out a piece (Fig. 2) of an Antennu- 

 laria, and hang it up vertically in the water, the 

 apical end b above and the root end a below, we 

 find that after a few days the root end a forms 

 little roots, r, which grow downward, 

 and the apical end, b, forms a new 

 apex, c. 



If we cut out a similar piece and 

 hang it upside down (Fig. 3), the 

 root end a, which now is above, 

 forms a new apex, ac, and the apical 

 end b, which is below, forms roots. 

 In such an apex the arrangement 

 of organs is just the same as in the normal 

 animal, namely, branches growing obliquely 

 upward bearing polyps on their upper sides. We 

 are, therefore, able to substitute a root for an 

 apex and an apex for a root. I have called this 

 substitution of one organ for another hetero- 

 morphosis. If we place the cut-out piece of 

 Antennularia horizontally instead of vertically, 

 something still more remarkable happens, namely, 

 the branches on the lower side suddenly begin to 

 grow vertically downward, and the outgrowing 

 parts are no longer branches but roots, ;-/', Fig. 4. This we can 

 prove by their physiological reactions, for the roots fix them- 

 selves to the surface of solid bodies, for instance, the glass of 



r 



Fig. 2. 



Fk;. 3. 



