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The completion of a full series of cellular associations, between two successive appearances of the same stage, along the length of the seminiferous tubule, is known as the wave of the seminiferous epithelium. Thus, for Rattus norvegicus, the wave of the seminiferous epithelium is “a series of adjacent segments, each of which includes the 14 possible types, in addition to any segment which is involved in modulation” (Perey et al. 1961). ‘Modulations’ are irregularities caused by temporary and spatially limited reversion or inversion of the numerical order in successive occurrences of stages in the cycle of the seminiferous epithelium (Perey et al. 1961; Dietrich et al. 1986). In R. norvegicus, the numbering of the stages in the wave decreases from the rete testis, into which the loop of the seminiferous tubule opens, at both ends (Perey et al. 1961). Modulation appears to be a frequent occurrence in the waves, because only 20% of the waves are unaffected, while up to 17% have more than three modulations in R. norvegicus (Perey et al. 1961). The phenomenon of modulation has not been reported in birds, apparently because of the anastomotic nature of the seminiferous tubules in this class of animals (Lin and Jones, 1990).