Spermatogenesis and chromatin condensation in male germ cells of sea cucumber Holothuria leucospilota (Clark, 1920). Male germ cells in the testis of Holothuria leucospilota can be divided into 12 stages based on ultrastructure and patterns of chromatin condensation. The spermatogonium (Sg) is a spherical-shaped cell with a diameter of about 6.5-7microm. Its nucleus mostly contains euchromatin and small blocks of heterochromatin scattered throughout the nucleus. The nucleolus is prominent. Primary spermatocytes are divided into six stages, i.e., leptotene (LSc), zygotene (ZSc), pachytene (PSc), diplotene (DSc), diakinesis (DiSc) and metaphase (MSc). The early cells are round while in DiSc and in MSc cells are oval in shape. From LSc to MSc, the sizes of cells range from 3.5 to 4microm. LSc contains large blocks of heterochromatin as a result of increasingly condensed 17nm fibers. In ZSc, the nucleus contains prominent synaptonemal complexes but a nucleolus is absent. In PSc, heterochromatin blocks are tightly packed together by 26nm fibers and appeared as large patches in DSc. Heterochromatin patches were enlarged to form chromosomes in DiSc and MSc and then the chromosome are moved to be aligned along equatorial region. The secondary spermatocyte (SSc) is an oval cell about 4.5-5.5microm. Their nuclei contain large clumps of heterochromatin along the nuclear envelope and in the center nuclear region. Spermatids are divided into two stages, i.e., early spermatid (ESt) and late spermatid (LSt). The nuclei decrease in size by a half and become spherical; thus the chromatin fibers condensed into 20nm and are closely packed together leaving only small spaces in LSt. The spermatozoa (Sz), with chromatin tightly packed in the spherical nucleus with a diameter of 2microm and a small acrosome situated at the anterior of the nucleus. The tail consists of a pair of centrioles lying perpendicular to each other and surrounded by a mitochondrial ring, and an axonemal complex, surrounded by a plasma membrane.