It has been previously reported that the distal half of the long arm of the human Y chromosome shows a differential affinity for fluorescent acridine derivatives (Zech, 1969; Pearson, Bobrow & Vosa, 1970). By the use of this staining reaction, the Y chromosome can be detected in interphase nuclei of lymphocytes, cultured skin fibroblasts and buccal mucosal cells. This technique has also been used to demonstrate that the X chromosome pairs with the short arm of the Y in first meiotic prophase (Pearson & Bobrow, unpublished observation). This paper describes the appearance of the Y chromosome in the different stages of spermatogenesis.Cytological preparations for the examination of meiotic cells were prepared from testicular tissue of three normal adult males by the method of Evans, Brecon & Ford (1964). Some material was prepared without hypotonic treatment, particularly
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