i8o 



Heredity and Environment 



mosomes are the only portions of the germ cells which ful- 

 fil these conditions. The association, segregation and distribu- 

 tion of Mendelian factors and of maternal and paternal chromo- 

 somes are exactly parallel and it is not reasonable to suppose that 

 this remarkable coincidence is without significance. 



Furthermore there is much additional evidence that the chro- 

 mosomes are important factors in heredity and development: 

 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. McClung, Stevens, Wilson and others have dis- 

 covered that the determination of sex is associated with the pres- 

 ence or absence of a particular chromosome, the X or Y chromo- 

 some, in the spermatozoon which fertilizes the egg. If an egg 



FIG. 59. DIAGRAM SHOWING UNION OF FACTORS IN FERTILIZATION AND 

 THEIR SEGREGATION IN THE FORMATION OF GERM CELLS. With 4 pairs of fac- 

 tors (Aa, Bb, Cc, Dd) 16 types of gametes are possible as shown in the 

 two series of small circles at the right. (After Wilson.) 



