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The male gamete. The typical sperm of most animals has a head containing the nucleus and acrosome, a middle piece with the mitochondria, and a tail with the 9 + 2 microtubule pattern (see illustration). Sperm, as well as the acrosome shape, varies with the species. The nucleus consists of condensed chromatin (deoxyribonucleic acid, DNA) and histone proteins. The acrosome, which is derived from the Golgi complex, contains hydrolytic enzymes, that is, hyaluronidase capable of lysing the egg coats at fertilization. Actin molecules which aid in the interaction between sperm and egg are found in the area between the acrosome and nucleus. The mitochondria in the middle piece apparently provide the energy necessary for the motility created by the tail. The tail has a central core, or axial filament, made up of nine double tubules and two central tubules. See Cilia and flagella
Many groups, including nematodes, myriapods, and crustaceans, have atypical sperm which lack a flagellum and are presumably nonmotile. The sperm of Ascaris is round and moves by ameboid means. The crustacean sperm have a large acrosome of several components. In the anomurans a middle with mitochondria and arms filled with microtubules precedes the nucleus. In the true crabs the nucleus forms arms which possess microtubules and also surrounds the many component acrosomes. The nucleus of crustacean sperm does not condense with maturation as it does in typical sperm mentioned above. See Spermatogenesis