THE CHROMOSOMES 



95 



research had made the fruit fly, DrosopMla mdanogaster, the most impoi- 

 tant animal in that field, yet further progress in the correlation of the 

 genetical phenomena with chromosome behavior seemed to be blocked 

 along some lines by the small size of the fly's chromosomes. All this is 

 now changed. The giant salivary-gland chromosomes have characters 

 that render them almost ideal for the purpose, and as a result the science 

 of c>i^^ogenetics has received a groat stimulus. They arc also furnishing 

 xaluable new information regarding the fundamental structure and 

 composition of chromosomes. It is fortunate that the cells containing 



Fig. 68. — Arrangement of salivary 

 chromosomes in the nucleus of Drosophila 

 melanogaster. The arms of the chromo- 

 somes extend from the chromocenter 

 formed by their heterochromatic portions. 

 (After T. S. Painter.) 



«»;-•- «• 





M^l 





Fig. 69. — Nucleus of living cell in 

 salivary gland of a fly (Chironomus) , show- 

 ing giant chromosomes. (After H. Bauer.) 



them are so located that the skilled investigator can prepare them for 

 study by simple and rapid methods. 



The first striking character of salivary-gland chromosomes is their 

 great size (Fig. 67). They are usually between 70 and 110 times as long 

 as the chromosomes in the oogonial cells. When moderately stretched 

 for the study of certain structural details they may be 150 times the length 

 of the oogonial chromosomes, the longest chromosomes of the complement 

 then reaching a length of about half a millimeter. In some flies all the 

 chromosomes lie well separated in the nucleus. In others, including 

 Drosophila melanogaster, the heterochromatic portions about their 

 kinetochores are all grouped into a single mass known as the chromoccitlcr 

 (Fig. 08). 



