MATERIALS OF HEREDITY II 



twice as many chromosomes as either of the parents. How 

 does it happen that this is not true? How does it happen 

 that the child has only the same number of chromosomes 

 as each separate parent? 



This was long a puzzle. But study with the microscope 

 answered this question. The two parents do not give all 

 of their chromosomes to each germ cell, but only half of 

 them. In forming the germ cells by division of the cells of 

 the parent, a very remarkable thing occurs, a thing that 

 does not occur at any other cell division. In all other cell 

 divisions each chromosome splits into two new ones, so 

 that each new cell gets the same number of chromosomes 

 that the parent cell had (figure 2). But in this division to 

 form the germ cells, the chromosomes do not split. In- 

 stead, half of them go into one of the new cells, half into 

 the other. And so the germ cell, as it is finally formed, has 

 not the usual number of chromosomes, but only half of 

 them. This is true for both the sperm and the ovum. 



And so when the sperm and the egg unite, the two pro- 

 duce a new cell that now has, restored, the usual number 

 of chromosomes. The sperm cell and the egg cell are really 

 only half-cells, so far as their chromosomes are concerned; 

 their union produces a complete cell again, and this now 

 develops into a new individual — every one of its cells con- 

 taining the full number of chromosomes — half from his 

 father, half from his mother. 



What Is the Function of the Chromosomes? — Imagine 

 an investigator who, working with infinite pains and taking a 

 very long time, discovers all these strange facts. Naturally, he 

 is stirred, he is excited. He asks himself, what can these things 

 be? What is their function? What do they accomplish by 

 all these complex evolutions, by splitting themselves accu- 

 rately into two pieces to form new chromosomes, by re- 

 ducing their number to half, when a new individual is to 



