The patterns of H3-thymidine labelling in the salivary gland chromosomes of Drosophila melanogaster were studied after alkali-urea treatment. Four essentially different labelling patterns were found: sparse labelling mainly or only in the central axis; spot labelling with almost continuous label in the axis and around it; labelling of whole chromosomes; spot labelling without appreciable label of the central axis. The possible chronological order for these patterns was suggested on the basis of earlier results. The labelling patterns were also interpreted according to the loop model of the lampbrush stage.
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