SYNOPSIS.All stages (except conjugation) of the life cycle inAcineta tuberosa(Suctoria) were studied in the electron microscope. Budding starts with the invagination of a small pellicular area near the opening of the vacuole and bearing rows of barren basal bodies. Some of these basal bodies are incorporated into the swarmer‐anlage and give rise to 8 rows of cilia, while the other basal bodies remain barren on the mother cell's side of the brood pouch. They will provide their own genetic continuity and later give rise to the swarmer of the next generation. Kineties are not spaced on the growing swarmer‐anlage until their cilia are fully grown. Further details on the stalk‐forming crganelle and general features of the development of the larvae are given.Metamorphosis starts with the formation of basal disc and stalk. The stalk‐forming material is released thru scopular pores into a tube‐like mold formed by the scopula after having been pulled inward. Later, scopula pores spread from the scopular area proper and move upward. On their way to the apical part of the animal the cuticle is secreted. The differentiation of tentacles is described and new observations regarding a possible mode of haptocyst formation are given.The results are discussed with respect to the morphogenetic potentialities of cortical structures i