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. 



a lUnli \ i: Ml \ 



a 





i\i ll 





<^»'*V*'^ 



<fe: 







./ 



/ 





Fig. 24. — Division figures in egg oi Ctenolabrus 9 . fertilized by Fundulus cf . 



In DrosopMla sl 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 



