For each cell tracked, STAGES determines its position in the cycle
after the user-specified number of days.  This can be either forward
(positive days) or backward (negative days) in time.

Since it may not be known exactly how far along in a stage an observed
cell is, STAGES determines the cycle position reached from 4 points
in the initial stage:  at the beginning, at the 1st third, at the 2nd
third, and at the end.

The output table shows the cycle position reached for each initial
cell, had that cell started in the first third, middle third, or
last third of its initial stage.
"Pre-Entry" or "PreEnt" means that the given number of days was
greater than the time the cell had existed in the cycle.  Since the
exact starting points of spermatogenesis cannot be identified
precisely, approximate points of entry are used.  Cells prior to the
point of entry are considered stem cells.  Note that the lineage for
undifferentiated spermatogonial cells is not completely understood.
The cells shown in the output represent the positions that other
cells of the initial stage would have tracked to in the given time.

"Epididymis" means that spermiation has occurred, and the cell is
moving through the epididymis.  Note that the transit times used are
only estimates; the actual transit time depends upon several factors,
including ejaculation frequency, relative sizes of testis and
epididymis, and age of the animal.  There will be variation between
animals and experimental groups; therefore, care must be taken when
interpreting the sperm transport data.

"Ejaculate" or "Ejact" means that the given number of days was
greater than the time required for spermiation to occur.