THE CELLULAR BASIS 165 



the egg and sperm cells the normal mimber 

 of chromosomes is reduced by one-half, and 

 (4) in fertihzation 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 

 of that individual's germ cells. Such inherit- 

 ance is known as Mendelian and will be 

 treated at length in the next chapter, but it 

 may be said here that the association, distribu- 

 tion and segregation of Mendelian factors 

 and of maternal and paternal chromosomes 

 are exactly parallel. This is strong evidence 

 that these factors are associated in some way 

 with the chromosomes. 



There are also certain special reasons for 

 considering that the chromosomes are import- 

 ant factors in heredity and development: (5) 

 Boveri has studied the abnormal distribution 

 of chromosomes to different cleavage cells in 

 doubly fertilized sea urchin eggs and has 



