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Abstract
Chromosome preparations of the sea urchin Clypeaster japonicus were made from early embryos. To obtain well spread chromosomes in air dried preparations, fertilization membranes were removed, embryos treated with colcemid (0.1-2.0 μg/ml) were dissociated into their component cells by Ca-Mg-free sea water, and dissociated cells were swollen with hypotonic (0.5 M) KCl. Thanks to adequate spreading of the cytoplasm, only the chromosomes stained well with Giemsa.