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 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.