THE CHROMOSOMES 



6 7 



number of chromosomes in every cell. The resulting 

 mechanism would no longer be able to develop normally, 

 if at all. 



The whole process may be represented by the fol- 

 lowing diagram, in which A, A' , and again B, B' ', are 

 homologous chromosomes, derived from different par- 

 ents and representing similar structures, but not pre- 

 cisely alike. At the reduction division, because one of 

 each pair goes out, we get gametes of four sorts, AB, 

 A'B, AB', A'B'. If these are sperms, uniting with a 

 similar series of eggs, we may have : 



The first four zygotes are all alike, and are heterozygous 

 (cross-bred) for both sets of factors carried by the 

 chromosomes ; the fifth is unlike the others, being 

 homozygous (pure-bred) for both sets of factors ; but if 

 dominance is complete, it will appear like them. 



Sperm cells (gametes) from male 



.jerms fail 

 B' 



' A A' \''^^ ~~" aT1C * s P ens h 



B BjSpermatogenes,* E^cell 



m fern ale 

 f A' B ' 



5permatocyte divisjon%^^ 

 the chromosomes are 

 separated into two sets, 

 but do not divide 



A - A' Ylvfo pairs of 



f homologous 

 B - B'J chromosomes 



FIG. 14. 



Polar bodies 

 (Which perish) 



Germ plasm 



Oiicyte division: half 

 the chromosomes will 



Zyiotes divide to fbrtn an individ- 

 TiaJ: the chromosomes also divide 

 (mitosis). Some of the cells thus formed 

 are set aside as < germ celk^emi plasm). 



Drawing by W. H. Schanck 



