222 



The Nature of and Changes in Genes 



chromatin of cold-treated plants, for comparable results were not 

 found in other species of Fritillaria. 



The difference in the amount and position of the heterochro- 

 matin in the chromosomes of various species of Drosophila is 

 very interesting. In the most familiar one, and the one that 

 we have cited most frequently, D. melanogaster , about one-third 



Fig. 69. Chromosomes in metaphase of mitosis in the pollen grains of a 

 triploid Fritillaria pudica. The plant was subjected to cold for over three 

 weeks. As a result, the production or distribution of nucleic acid is hind- 

 ered and the available nucleic acid is seized by the euchromatin. The 

 heterochromatin in such chromosomes stains much less heavily and is rep- 

 resented by the light areas in the figure, (From Darlington and La Cour 

 in the Journal of Heredity. Courtesy of Dr. C. D. Darlington.) 



of the X chromosomes and up to one-sixth of each arm of each 

 autosome is heterochromatic. In the salivary glands, all the 

 chromosomes are united to the chromocenter, which is made up 

 almost equally of material from the X chromosome and the 

 two large, V-shaped autosomes. The chromosomal picture of D. 

 simulans is practically the same. In D. virilis, a little less than 

 half the length of each chromosome is heterochromatic, and all 

 chromosomes contribute almost equally to the chromocenter. 

 Very different from these is D. hydei. In this species the X 

 chromosome is V-shaped; one arm appears to be entirely hetero- 



