92 The Mechanistic Conception of Life 



may speak of positive stereotropism in the case of the root, and 

 of negative stereotropism in the case of the polyp. 



Here, too, we may ask whether the contact with foreign 

 bodies, which in these experiments determines the arrangement 

 of the various organs, may not have the same effect in the 

 natural development of the organism. I believe that such is 

 the case. Negative stereotropism forces the polyps to grow 

 away from the ground into the water, and hence parts sur- 

 rounded by water form polyps only. Positive stereotropism 

 forces roots in contact with the ground to hold to it, hence 

 parts in contact with the ground give rise to roots only. Thus 

 it happens that, under ordinary circumstances, in this animal 

 we find roots only at the base where it touches the ground. In 

 other hydroids the place of origin of the different organs is 

 determined by light, and in others we find more complicated 

 relations. 



It may appear from the foregoing that such cases of hetero- 

 morphosis are confined to hydroids, but such is not the case. 

 We find similar cases in Tunicates. Ciona intestinalis, a solitary 

 ascidian, has eye-spots around the two openings into the 

 pharyngeal cavity. If we make an incision eye-spots are 

 formed on both sides of the incision.^ 



II. POLARIZATION 



While the foregoing experiments were in progress, I observed 

 that in many animals I was unable to produce any kind of 

 heteromorphism. These animals showed, in regard to the 

 formation of organs, a phenomenon with which we are familiar 

 in a magnet. If a magnet is broken into pieces, every piece 

 has its north pole on that side which in the unbroken magnet 

 was directed toward the north. Likewise, there are animals 

 every piece of which produces, at either end, that organ toward 



> Since this was written phenomena of heteromorphosis have been produced 

 in many animals. Herbst found that in crustaceans an antenna covild be caused 

 to be formed in the place of an excised eye, Van Duyne, Bardeen, and Morgan 

 observed phenomena of heteromorphosis in Planarians and so on (1912). 



