INTRODUCTION 13 



the snake. Other types of reactions such as reduction or combi- 

 nations with other compounds than oxygen will not produce the 

 same result. Even though a cone is made of mercury sulpho- 

 cyanate and that cone is burned a serpent will not be formed 

 unless the cone burns from the tip. It must not burn at the same 

 rate on all its surface else some other kind of monster will grow 

 out of it. This means that certain physical conditions also must 

 be met. So the scientist pictures the growth of a zygote into an 

 individual as depending upon a particular kind of living stuff 

 carrying on a particular series of chemical reactions and physical 

 processes which work out into the characteristic structure of the 

 individual.* The chemical constitution of the living stuff in a 

 zygote is, of course, much more complex than that of the mer- 

 cury sulphocyanate in the cone and the chemical reactions and 

 physical processes concerned in the growth of a zygote into the 

 simplest creature are infinitely more numerous and complicated 

 than those which take place in the oxidation of the sulpho- 

 cyanate. 



The discoveries within the last decade of the remarkable in- 

 fluence of the secretions of various glands upon differentiation 

 in animals are a wonderful and fascinating record of the prog- 

 ress in our knowledge of differentiation. Wonderful as these 

 discoveries are they are but a beginning. They do not tell us 

 much of the differentiation in the many living things which 

 have no such glands nor the causes of the differentiation in the 

 glands themselves. 



You can readily perceive, however, that the statement that 

 growth takes place by a division of cells, an enlargement of the 

 cells so produced, and a differentiation of these cells into perma- 

 nent form is only a skeleton outline of the process of growth. 



Cell division, as it usually occurs, is a complex process, par- 

 ticularly for the nucleus. The series of drawings in Figure 9 il- 



* These chemical reactions and physical processes may be modified by the ex- 

 ternal environment just as the burning of the mercury sulphocyanate may be af- 

 fected by the character of the gas supplied. 



