THE CHROMOSOMES 



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 r , 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 



^ /^^"\Most sperms fail 



Germ plasm IA B'jto findan egg cell. 



.-^~ ^te--' and so perish . 

 jSpermatogenesis E cell 



Oogen- 



Obcyte division: half 

 the chromosomes will 

 be rejected as poJar 

 bodies . 



Spermatocyte dmsiore 

 the chromosomes are 

 separated into two sets. 

 but do not divide 



A - A' |Two pairs of * 



/ homojogous 

 B - B'J chromosomes 



om female 



5permand<gg 

 unite (rertil- / 

 -ization_) /-' 



Polar bodies 

 ' B (which perish) 



Germ plasm. 



Zygpte Zygotes divide to form an individ- 



ual' the chromosomes also divide 

 (mitosis) Some of the cells thus formed 

 are set aside as^rm cells germ plasm). 



FIG. 14. Drawing by W. H. Schanck 



