lO GENETICS 



cell. Then the halves each enlarge to form a complete 

 chromosome; these swell up and become little vesicles, which 

 are pressed together and form a new nucleus for each cell 

 (figure 2). And when this new cell comes to divide, these 

 vesicles form by condensation each a new chromosome like 

 the one from which it came; then these chromosomes go 

 through this complicated process of division again. 



This division of the chromosomes is very precise; each 

 chromosome produces two new ones exactly like itself. 

 Often the different chromosomes in the cell differ greatly 

 in size and form. When each one divides, the two new ones 

 produced are like the particular parent chromosome, in 

 size and in form. 



All this appeared marvelous to those who observed it 

 under the microscope, and it still appears so to anyone 

 who follows it in any organism. What does it all mean? 

 What is it that is occurring here? 



To pursue the observations further, certain other things 

 that appeared still more remarkable were discovered. In 

 any kind of organism, every one of the millions of cells 

 has a set of these chromosomes. And the number is constant 

 throughout the organism; every cell has the same number. 

 Each kind of creature has its own characteristic number, 

 varying from two, in some species, to hundreds in others, 

 but for each organism the number is constant. The same 

 number of chromosomes that is found in the cells of either 

 of the parents is later found in the cells of his offspring — 

 with certain modifications that will be mentioned later. 



But this fact, that the child has in his cells the same num- 

 ber of chromosomes as either single parent, seemed to the 

 early investigators remarkable. For the sperm is a cell 

 from one parent, the ovum a cell from the other, and the two 

 unite to form the cell that becomes the child. One might 

 expect that when the two united, this would give the child 



