Biology 689 



of fact, it is a necessary and inevitable property of the lowest 

 forms of life. The simplest way, perhaps, of realising the in- 

 evitability of regeneration is to remember that the appearance 

 and structure of any animal or plant is the product of the balance 

 between its own constitution and the environment that surrounds 

 it. This is true, of course, for many portions of matter that are 

 not alive. A drop of mercury on a saucer, for instance, assumes 

 approximately the shape of a sphere, because of the laws of sur- 

 face tension between mercury and air, and mercury and china ; if 

 we cut it in two, each half becomes a separate sphere. If a drop 

 of mercury were an organism, we should say that its typical form 

 was spherical, and that any fragment of the whole was capable 

 of reorganising itself in the typical form. 



If we take a single-cell animal, and cut it into two or into 

 many parts, each part, providing it is above a certain minimum 

 size and contains the whole or part of the nucleus, will readjust 

 itself until it is once more in a state of equilibrium in other 

 words, until it is of the normal shape and structure of the species. 

 Furthermore, the miniature animals thus produced, unlike the 

 miniature drop of mercury, are capable of growth; so that in 

 these simple forms regeneration is the necessary outcome of the 

 two faculties of reorganisation and growth. 



Even in many multicellular animals, a similar unlimited 

 power of regeneration is to be found. Any piece of the stem of 

 a polyp, any fragment of a Planarian flat-worm, will in almost 

 all cases reorganise itself into a new whole. 



Producing a New Head 



In these larger organisms, the mechanism of the process is 

 somewhat complicated. It appears that if a piece of flat-worm, 

 for instance, be separated from the rest, it will first produce a 

 new head-region; and that the head-region, once formed, will 

 control the rest of the piece, so that all the rest of the parts of 

 the body are formed in order from head to tail. One may say 



