In gonad-bearing animals gametogenesis can be divided into three main
phases, During embryonic development the primordial germ cells move to
wards the gonadal primordia, A long, intra-gonadal phase follows durin
g which the germ cells grow and differentiate, Mature germ cells are f
inally released from the gonads and brought to the exterior, Thus, ger
m cells are successively motile, non-motile and motile again, This com
plex life history is given here a simple evolutionary interpretation,
The basic assumption is that primitive Metazoa already had germ cells,
but no gonads to harbour them, Higher animals acquired gonads, which
sequestered the germ cells, thus creating the temporary confinement ex
perienced by germ cells in most present-dag Metazoa. This evolutionary
scheme may explain why several steps of germ cell differentiation are
totally or partially independent of the gonads, These steps presumabl
y existed in primitive, gonad-free Metazoa, and conserved their autono
my in higher animals.