The method of ultrathin sections of unsquashed salivary gland polytene chromosomes of Ch. thummi was applied to their ultrastrual mapping. There was a good agreement between electron micrographs and Hägele's light microscopic map (1970) with respect to the pattern and number of bands. 94% of bands were identified in larval and prepupal chromosomes. In Ch. thummi, band thickness varied from 0.05-0.5 mum. Most characteristic were 0.2-0.3 mum bands. Morphologically, bands were classified as: continuous (frequently with holes and gaps), discrete, dotted and continuous-discrete, discrete-dotted. Band morphology is related to band size, such that smaller bands, as a rule, were also dotted. Bands beginning to puff likewise became dotted. Interbands in unsquashed chromosome sections were from 0.05-0.15 mum. The smallest interbands contained only fibrils, in the larger interbands few granules could be observed. This makes interbands distinguishable from a typical puff with many such granules.
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