Résumé : During the development of a fresh water sponge from its gemmules, most cell types originate from the undifferentiated archaeocytes through a few divisions, whereas each choanocyte chamber, composed of several tens of choanocytes, arises from a single archaeocyte through repeated mitoses. This process was studied on gemmules incubated in various concentrations of hydroxyurea. A concentration of 100 μg/ml postponed the hatching by about 2 days, and blocked the differentiation of the choanocytes and the morphogenesis of the aquiferous system. The resulting organism was a hollow dome of pinacoderm, stretched on spicules, the bottom of which was strewn with embryonic archaeocytes. After washing and incubation in mineral medium, the sponge differentiated its choanocytes and achieved normal development. The incorporation of 3H thymidine into DNA was compared throughout the development of normal and hydroxyurea treated gemmules. Hydroxyurea delayed the first peaks of incorporation and abolished the large peak that normally occurs around 90 hr, just before the formation of choanocyte chambers. When added after 96 hr incubation, hydroxyurea did not affect the differentiation of the choanocytes. These results suggest that the differentiation of the choanocytes and the further morphogenesis of the aquiferous system depend on the repetitive divisions of the archaeocytes that normally occur around 90 hr. Furthermore, hydroxyurea blocked sponges provide a suitable source for the isolation of pure populations of embryonic archaeocytes.