Lampbrush chromosomes which are chromosomes in meiotic pro- phase with laterally projecting loops, have been observed in oocyte nuclei of several vertebrate, some invertebrate species, and in spermato- cyte nuclei of Drosophila species. Especially large lampbrush chromo- somes occur in oocytes of the urodele Amphibia and in spermatocyte nuclei of Drosophila hydei and some other species belonging to the hyde i- subgroup. In Amphibia the premeiotic chromosomes show a chromomeric organization and a pair of lateral loops projects from each chromomere. Often the loops show a thin axis and a matrix consisting of many radial projecting fibers attached to it. Moreover, the loops are often asymmetric. One of the insertions of the loops in the chromomere may have long fibers and as one follows around the loops from this end, the projecting fibers become progressively shorter and then disappear so that the second end of the loops is bare of fibers. Some of the large loops show morphological peculiarities in length or thickness, or in the nature of their matrix. These characters are constant and locus specific and have allowed to establish loop maps for the genome of several Amphibian species.
Lamp- brush chromosomes are highly extensible. Breakages after stretching beyond the elastic limits occur always transversely accross chromo meres in such a way that a pair of lateral loops span the break. The axis of the loops appears to contain DNA and there is evidence that the DNA con- sists of only one double helix in each loop.