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The ultrastructural and histochemical changes of the sex chromosomes (sex vesicle) during meiosis in the male mouse have been sequentially studied. The sex vesicle is first identified at zygotene and lasts up to late diplotene. The sex vesicle is attached to the nuclear membrane from early pachytene, and it is separated from other intranuclear structures by a clear space from the same stage. The nucleolar part of the sex vesicle develops from middle pachytene and attains its maximum size at late pachytene, when it contains several special regions. The nucleolar part becomes separated from the chromatin part before the end of diplotene. The packing of the chromatin in the sex vesicle remains essentially the same up to late diplotene, when some further condensation of the chromatin occurs. The cores of the sex vesicle are double except at middle pachytene, when a structure identical to a synaptinemal complex …