MECHANISM IN SEOREOATION 53 



all embryonic cells, have three chromosomes each (Fig. 

 64), showing that when an egg starts with three chromo- 

 somes, this number is retained through all subsequent 

 divisions, despite the fact that after each division a rest- 

 ing stage intervenes. 



F'S 







a a' 





d' 







FiQ. 23. — Normal a, b, and reduced a', b', chromosomes of two species of Biston; and of 



hybrid c, c'. 



The evening primrose, (Enothera Lama/rchiana, has 

 14 chromosomes (reduced number 7). Individuals are 

 known in which there are 15 chromosomes. As a result 

 of accidental displacement at a division in a germ-cell, 

 possibly one cell came to contain an additional chromo- 

 some. Such a cell combining with a normal one, at f ertil- 



