234 GENETICS 



How does it happen that the different parts of the body j 

 become diverse? Since all the genes are present in all the 

 cells, why do not all the cells change in the same way, in- 

 stead of becoming different? 



Not very much is known about this; it is one of the dark- 

 est questions of biology. But some things are known about 

 it that are of great interest for the question as to the 

 effects of environment. Look at one or two experiments. 



The original single cell divides into two cells A and B. 

 These two cells later produce the two halves of the body. 

 That is, one produces the right half, the other the left half. 



Why does the cell A produce only the left half of the 

 body instead of the whole body? Try separating the two 

 cells. This can be done, with some difficulty, in certain or- 

 ganisms. Now we find that this cell A, when separated, pro- 

 duces, not the left half of the body, but the entire body, 

 an entire individual; and the right-hand cell B will do the 

 same. 



So it is clear that what each cell produces depends on 

 its environment, on its relation to the other cells. Separate 

 the two; then the genes in each produce an entire indi- 

 vidual, in place of a half individual. 



This sort of thing turns out to be typical for develop- 

 ment. Every cell contains as a rule all the genes, and so 

 far as genes are concerned could produce an entire indi- 

 vidual. But after a while, through the continued activity of 

 the genes, many different substances have been manufac- 

 tured from the cytoplasm, and are located in the different 

 cells. Things have now changed so much, and have become 

 so fixed, that the genes can no longer start anew from the 

 beginning. At such a later stage therefore a single sepa- 

 rated cell will no longer produce an entire organism, or any 

 required part of the organism, although it still contains all 

 the genes. With age the cells become fixed in their ways, as 

 old individuals do. 



