170 HEREDITY AND ENVIRONMENT 



naive in the view that this most general pro- 

 cess can be localized in one specific part of the 

 cell, something which recalls the long-past 

 doctrine that the life was located in the heart 

 or in the blood, or the ancient attempts to find 

 the seat of the soul in the pineal gland or in the 

 ventricles of the brain. 



Nevertheless there are certain general rea- 

 sons for assuming that the chromosomes are 

 important factors in heredity and differentia- 

 tion: (1) they come in approximately equal 

 numbers from the father and the mother, (2) 

 one-half of each of the maternal and paternal 

 chromosomes is distributed to each cell of the 

 developing organism, (3) in the formation of 

 the egg and sperm cells the normal number 

 of chromosomes is reduced by one-half, and 

 (4) in fertilization the normal number is re- 

 stored by the union of the chromosomes of 

 the egg and sperm. It is a remarkable fact 

 that the determiners or factors of certain in- 

 herited characters come in equal numbers 

 from both parents and that in spite of their 

 ultimate association in an individual they may 

 be separated or "segregated" in the formation 



