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The duration of the cycle of the seminiferous epithelium is a fundamental parameter of spermato- genesis in mammals (Heller & Clermont, 1964). Its measurement in the field vole (Microtus agrestis) could clarify the way in which seasonal changes in spermatogenesis in this species (Clarke & Forsyth, 1964) are brought about. Furthermore, in our laboratory colonies puberty in M. agrestis occurs at about 6 weeks of age, whereas in the bank vole (Clethrionomys glareolus) it is 5-6 weeks later (Greig, 1968). The duration of the seminiferous epithelial cycle has therefore also been estimated in C. glareolus to determine whether this dissimilarity is associated with a difference in the duration of the cycle of the seminiferous epithelium. As in other species (Roosen-Runge, 1962), spermatogenesis in M. agrestis (Grocock, 1972; Grocock & Clarke, 1975) and C. glareolus (Grocock, 1972) can be divided into 8 stages whose frequen- cies may be determined. The duration of a cycle of the seminiferous epithelium can be established by combining these data with observations on the progression through the epithelium of germ cells labelled with (3H)thymidine (Clermont & Trott, 1969). The complete process of spermatogenesis, from the first division of spermatogonia in stage 5 to the final release of spermatozoa in stage 8, comprises approximately 4\m=.\6 cycles (Grocock, 1972).

Duration of spermatogenesis in the vole (Microtus agrestis) and bank vole (Clethrionomys glareolus)