56 SCIENTIST 



What has happened to make biology so attractive to the 

 orderly sort of mind? In a word, the probability has 

 emerged that many — if not all — of the puzzling character- 

 istics of life will find their explanation in the shape of the 

 very large molecules which form the genetic material of 

 the chromosomes. These molecules are known to be long 

 linear chains of substances called purine and pyrimidine 

 bases (linked together with a sugar and some phosphoric 

 acid). Ordinarily there are four different bases available 

 and it is the order in which they are put together that 

 apparently determines just about everything that happens 

 in the making of any Uving organism. 



Roughly two hundred years of observation have pro- 

 vided a good descriptive knowledge of what happens as 

 a single egg cell grows, divides, and differentiates into skin, 

 gut, bones, muscles, nervous system, and glands, but there 

 have been only vague clues as to how this intricate process 

 was guided and timed. It is clear enough that each stage 

 in the process starts at a particular point and stops when 

 the organ or system in question reaches its appropriate size. 

 In the case of man, certain stages like bone growth proceed 

 for as long as 20 years. Others, Uke the differentiation of 

 the nervous system, are finished shortly after birth. The 

 phenomena of adolescence wait 10-15 years even to get 

 started. But how does all this come about? What determines 

 the time at which some processes stop and others begin? 

 The key to understanding these really wonderfully coordi- 

 nated events now seems to lie in the genetic code which 

 also, of course, determines the hereditary differences among 

 species and between individuals within a single species. 



Less clear at present, but probable and provocative, is 

 the idea that certain responses to environmental influences 

 may depend on changes in the genetic code of specialized 

 types of cells. The development of immunity to infectious 



