Calendar of gametogenic development in the prepuberal male mouse.

The timing of the first meiotic cycle in the young male mouse was established by serial sacrifices and subsequent paraffin sectioning of testis and epididymis. Meiosis commences in mice 8 to 10 days old. The highest frequency of any one phase of meiosis was found at days 17 to 19, when 47 percent of the tubules were found to contain late pachytene.