THE CHROMOSOMES 83 



iiig stages of the nucleus and during their period of 

 active division. There is a great deal of evidence from 

 direct observation in favor of this view and there is 

 more evidence from the modern work in hereditv 

 that points in the same direction. This evidence can 

 not be considered here, but if it is granted that these 

 relations hold, then the behavior of the chromosomes 

 during maturation furnishes, as stated above, an ex- 

 planation of Mendel's laws. 



An example will illustrate this statement. If in 

 the four o'clock the elements for red flower color are 

 carried in the red parent by the two members of the 

 same pair of chromosomes and the elements for 

 white flower color are carried in the white parent by 

 two members of the same pair of chromosomes, the 

 germ-cells (ripe egg- and sperm-cells) will each 

 carry one of these chromosomes {fig. 36) . If the red 

 plant is crossed to the white, the pink hybrid will 

 have a red- and a white-bearing chromosome. 



When in the hybrid the germ-cells ripen, these 

 two chromosomes, being mates, will come together 

 as a pair and then separate at one of the two matura- 

 tion divisions, and half of the eggs will contain the 

 red-bearing chromosome and half will contain the 

 white-bearing chromosome. Similarly for the pollen 

 grains. Chance fertilization of any egg by any sperm 

 will give the combinations of chromosomes that 

 JNIendel's law of segregation requires. In other 

 words the known behavior of the chromosomes is 



