Action of the head activator as a growth hormone in hydra.

At the cellular level the head activator from hydra acts as a mitogen or growth hormone. It shortens cell cycle times by stimulating cells arrested in the G2 period to go through mitosis. This is true for continuously proliferating cell types like epithelial cells, gland cells, and interstitial cells, and for differentiating interstitial cells including those undergoing a last mitosis before differentiating into nerves or nematocytes.