54 



PHYSICAL BASIS OF HEREDITY 



ization, would produce a plant of the 15-cliromosome 

 type. Here again, the additional chromosome persists 

 as an individual element of the cell throughout subsequent 

 cell-generations. 



.>i>'*V"^ 



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pi I ii U I 



\)^:,"A,r::-/) 







^v^- 





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ill "!l,.v.v 





Hi 



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 



