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3A. above, on the dorsal side, the episomites, the double row of primitive or muscular segments; and 3B. below, on each side of the gut, the hyposomites, the two lateral plates that give rise to the sex-glands, and the cavities of which partly unite to form the body-cavity.
Immediately after their formation the two coelom-pouches of the Amphioxus are divided into several parts by longitudinal and transverse folds. They have a different future above and below. The upper or dorsal segments, the episomites, lose their cavity later on, and form with their cells the muscular plates of the trunk.
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