Spermatogenesis is a process of synchronized and gradual
transformation of undifferentiated germ cells into spermatozoa
over a period of time, extending from 12-70 days, depending upon
the species.  The germ cells advance (or, change) within the
seminiferous epithelium, with the older cells being associated
with younger ones in a specific cycle.  The cycle of spermato-
genesis begins with mitotic division of spermatogonia and proceeds
through meiosis, finally ending with the release of sperm.

This process has been divided into cellular associations, called
stages, that vary somewhat between species.  The time required for
one pass through all of the stages constitutes one cycle of the
seminiferous epithelium.  In some species, histologic sections
of the testis reveal that cross-sections of seminiferous tubules
will contain a uniform epithelium at only one stage of the cycle
(e.g., rat, mouse, hamster, bull, pig, rabbit, dog, sheep, horse).
In other species, the seminiferous tubular cross-section will
contain a mixture of cells from multiple stages, which is due
to a spiralled pattern of epithelial growth (e.g., man, baboon,
bird).  In testes containing uniform epithelia, the stages are
repeated in consecutive order along the length of the seminiferous
tubules.  Because the tubules are folded repetitively, a cross-
section of the testis will contain several hundred tubule sections,
each of which can be identified as one of the stages.