No. 641] SHORTER ARTICLES AND CORRESPONDENCE 573 



The cross-over classes are tlie red-bar and white-eyed male 

 flies. The percentage of crossing-over being 2,172, divided by 

 4,893, and the quotient, multiplied by 100, which gives 44.4 per 

 cent. The difference, then, in the cross-over values when the 

 linked factors entered in different ways was but 1.1 per cent., 

 which does not seem to be a signicant difference. 



J. D. Ives. 



ON COUNTING CHROMOSOMES IN POLLEN-MOTHER 

 CELLS 



The genetic study of hybrids between species with different 

 chromosome numbers and of certain mutants recpiires the count- 

 ing of many chromosome groups and raises tli<' question of the 

 best technique for the purpose. The staining qualities of aceto- 

 carmine. whicli has long been used for preliminary work, espe- 

 cially by zoologists, are considerably improved by a trace of 

 ferric salt. ( Holies Lee, in his well-known mamial, gives for- 



over iron htematoxylin.) 



by heating a 45 per cent, solution of gliici;il ai-ctir acitl to boiling 

 with excess of ^powdered carmine, cnoliim- and filtering.^ The 



drop of this until it changes sliiihily inward bluish red. An 



moved, and a large thin coverglass (22 by 50 mm.) applied, 

 nsing the minimum of liquid. The edgf>s are sealed witli va.se- 

 line. The preparation, if there is no excess of iron, may improve 

 for a day or two. 



Iron Aceto- carmine 2. — To a quantity of aceto-carmine a trace 

 of a solution of ferric hydrate dissolved in 45 per cent, acetic 

 acid is addeil until the liciuid becomes bluish red, but no visible 



is then added. The anthers aiv teased out with nickel instru- 



