A parallel between development and evolution: germ cell recruitment by the gonads.

In gonad-bearing animals gametogenesis can be divided into three main phases. During embryonic development the primordial germ cells move towards the gonadal primordia. A long, intra-gonadal phase follows during which the germ cells grow and differentiate. Mature germ cells are finally released from the gonads and brought to the exterior. Thus, germ cells are successively motile, non-motile and motile again. This complex 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 experienced by germ cells in most present-day Metazoa. This evolutionary scheme may explain why several steps of germ cell differentiation are totally or partially independent of the gonads. These steps presumably existed in primitive, gonad-free Metazoa, and conserved their autonomy in higher animals.