THE CELLULAR BASIS 171 



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 

 found evidence that the hereditary value of 

 different chromosomes is different. (6) Mc- 

 Clung, Stevens and Wilson have discovered 

 that the determination of sex is associated 

 with the presence or absence of a particular 

 chromosome, the X chromosome, in the sper- 

 matozoon which fertilizes the egg. If an 

 egg is fertilized by a sperm which lacks the 

 X chromosome a male is produced, if fertilized 

 by the other type a female results. (7) Fin- 



