Action of the head activator on the determination of interstitial cells in hydra

In addition to its role as a growth hormone (preceding paper), at the cellular level the head activator functions as one of the substances which control the determination of uncommitted stem cells in hydra. In the presence of head activator the determination of interstitial stem cells to nerve cells is stimulated, the determination of interstitial cells to nematocytes is inhibited. The determination of interstitial cells to nerves occurs shortly before or in the very early S period of interstitial cells.