[The rules for electron microscopic mapping of polytene chromosomes in squashed preparations of Drosophila salivary...

The following rules of the polytene chromosome mapping at the submicroscopical level are proposed: 1) using anhydrous mixtures of alcohol with acetic acid for fixation of salivary glands which permits to avoid artifacts in the structure of large bands; 2) using sections 120-150 nm thick which improves the revealing of faint bands; 3) including in the analysis larvae being at different stages, since different bands display puffing at different regions; 4) reproducibility of banding pattern in serial sections of some chromosomes. The use of these rules permits to interpret more strictly the observed pictures of the polytene chromosome banding pattern and to avoid possible mistakes of the mapping.