54 



PHYSICAL BASIS OF HEEEDITY 



ization, would produce a plant of the 15-cliromosonie 

 type. Here again, the additional chromosome persists 

 as an individual element of the cell throughout subsequent 

 cell-srenerations. 



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Fig. 24. — Lagging and elimination of chromosomes in hybrid fish embryos. (After Pinney) . 



In Drosophila a female occasionally appears with two 

 X's and a Y-chromosome. There are several ways in 

 which this may arise, but the most common way appar- 

 ently is for an egg to retain both of its X elements. Such 

 an egg fertilized by a Y-bearing sperm produces an XXY 

 embryo. Such an embryo retains throughout the entire 



