200 



Biology in America 



that chromosomes carrying: the alternative determiners for 

 tallness and dwarfness pair with each other and later separate 

 in the maturation divisions, goin<? into different gerin cells. 

 Now it is manifestly impossible to loeate directly the deter- 

 miners for any character in any particular chromosome, or 

 in the chromosomes at all, for that matter. The direct 

 analysis of the chromosome is as yet impossible. Nor can we 

 prove directly that the paired chromosomes separate from 

 each other in the maturation divisions, instead of retaining 

 their paired character and dividing equally. 



We have however certain indirect evidence which strongly 

 supports our assumptions. In many species of animals and 

 plants, notably among insects, the chromosomes differ mark- 



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Diagram Showing Eight i'ossible Distributions of Three Pairs of 

 Chromosomes in the Maturation of the Germ Cells 



After Morgan, Sturtevant, Muller and Bridges, ' ' The Mechanism of 

 Mendelian Heredity. ' ' 



By permission of Henry Holt and t'umininy. 



edly in size or shape. In the equatorial plates of most 

 mitoses these chromosomes are very definitely arranged in 

 pairs with similar members. But after the reduction division, 

 when the chromosome number is reduced by half, the result- 

 ing cells each receive similar groups of chromosomes, so that 

 each chromosome of one cell, has its exact counterpart in its 

 sister cell, with certain exceptions in the case of the sex 

 chromosomes to be noted later. 



Seemingly then we are warranted in our conclusion that 

 in the reduction division the members of each pair are 

 separated from one another and distributed to sister cells. 

 And further, since the male and female sex cells each con- 

 tain similar groups of chromosomes, it is reasonable to assume 

 that similar or "homologous" chromosomes from male and 

 female pair with each other in fertilization, remaining paired 

 until the next generation of germ cells matures, when they 

 separate to be recombined once more in the succeeding fer- 

 tilization. 



