Physiological Morphology 107 



determine the marking. We do not kaow what processes 

 determine the coloration of animals which owe their markings 

 to interference colors, but the task of deriving such a coloration 

 in the adult from a similar arrangement of molecules in the 

 germ plasm would prove too much even for a genius like 

 Huyghens, and without the possibility of such a derivation the 

 theory is of no use. 



3. The reasons why roots grow on the under side of the 

 stem of Antennularia and stems on the upper side can only be 

 given when the special physical and chemical conditions inside 

 the stem of Antennularia have been worked out. At present we 

 can only think of possibilities. It is possible that the hypotheti- 

 cal root substances of Sachs may have a greater specific gravity 

 than the substances which form stems, and therefore take the 

 lowest position in the cell. Since outgrowth can take place 

 only at the free surface of a stem or branch, roots can on this 

 assumption grow only at the under side and stems only at the 

 upper side of an element. But there are still other possibilities 

 which we must omit here. In the case of Margelis and other 

 hydroids, it might happen that contact with solid bodies pro- 

 duced an increase of surface in the touched elements in case they 

 contained specific root substances, while the opposite took place 

 in the case of elements containing polyp substances. The con- 

 sequence would be an increase in the surface of the roots if they 

 came into contact with solid bodies, while polyps only would 

 grow out in the opposite direction. I found, indeed, in some 

 forms at Naples that roots of hydroids which grew free in the 

 water began to grow much faster and to branch off more 

 abundantly when brought into contact with solid bodies. But 

 in these cases we must wait with our attempts at explanation 

 until the physical and chemical conditions for the form are 

 worked out. For the same reasons I will not go into a discus- 

 sion of the question of what determines the polarization of ani- 

 mals like Cerianthus. It may suffice to suggest the possibility 



