GROWTH AND REPRODUCTION 47 



one parent has true blue (recessive) eyes and the 

 other hybrid brown eyes, there will be in the long 

 run (in the offspring en masse, not in each individual 

 family) an apparently definite percentage of children 

 with blue eyes and with brown eyes. 



There is some reason to believe that the pecul- 

 iarities of cerebral organization that underlie in- 

 sanity are subject to Mendelian heredity. On a 

 priori grounds we should expect that some of the 

 offspring of insane parents would be exempt. In 

 cases where the offspring is implicated the nervous 

 abnormalities may appear either in full force or in a 

 modified form, as in a blend. In either case there 

 may be in the next generation a complete elimina- 

 tion of these nervous abnormalities in some of the 

 children. On the other hand, some of the children 

 may show a blend inheritance which is not elimi- 

 nated in the next generation. 



It seems probable that we may look for the expla- 

 nation of Mendelian heredity to the materials con- 

 tained in the nuclei of the germ cells, for here there 

 are found bodies which have a history in the life 

 cycle of the organism showing a striking parallelism 

 to the history of the "unit character," such as the 

 gray and white mentioned in the case of the trans- 

 mission of colors in mice. These bodies are well 

 known to biologists as chromosomes, the clearly 

 defined rods or loops in the nuclei of the egg and 

 sperm seen at the time of fertilization and seen 

 subsequently every time a cell divides in the course 



