CHAPTER I 

 INTRODUCTION 



1. Living organisms, as well as crystals, are characterized by a 

 definite shape which is primarily determined by the chemical 

 nature of their material. Both types of forms grow, but the 

 mechanism of growth is different in the two types. Crystals 

 grow in supersaturated, undercooled solutions of the molecules 

 of which they are composed. Living cells grow in solutions of 

 low concentrations of simpler compounds than those of which 

 their cells are composed, and the growth of living cells is preceded 

 by a synthesis of the large insoluble molecules of the cytoplasm 

 and of the nucleus from the comparatively small molecules, 

 such as amino-acids, sugars, etc. 



It is very interesting that crystals and Hving organisms have 

 also the phenomenon of regeneration in common. When a muti- 

 lated crystal is put into a supersaturated, undercooled solution 

 of its substance, it restores its old form, and the same phenomenon 

 of repair or restoration of the old form occurs in mutilated living 

 organisms. The mechanism of regeneration is, however, also 

 entirely different in the two cases, for the reason that the mechan- 

 ism of growth (which gives rise to regeneration) is different. 



Let ahcd (Fig. 1) be the side of a cubical crystal such as 

 NaCl. In a supersaturated, undercooled solution of NaCl, 

 the crystal will grow slowly, always retaining its cubical form 

 because the rate of growth is exactly equal at each element of 

 the surface. This rate of growth is a relative minimum at the 

 normal surfaces of a crystal. When now the crystal is muti- 

 lated, e.g., when the angle at a is cut off so that the form aia^- 

 hdcai results (Fig. 2), the old normal surfaces, 026, hd, dc, and cai 

 continue to grow at the old minimal rate. The rate of the growth 

 of the new surface aya-i is, however, greater than that of the old 

 normal surfaces, and this difference in the rate of growth depends 

 on the angle of the new surface with the old surface where the 

 growth is a minimum. As a consequence of this greater rate of 

 growth of the new surface ciici-y, this surface is bound to eliminate 



1 



