Embryogenesis in hydra includes a variable period of dormancy; and this period, as well as subsequent stages through hatching, takes place within a thick cuticle that hinders observation. Thus, although the early stages of development have been well-characterized qualitatively, the middle and later stages are only poorly understood. Here, we provide a detailed description of the stages of embryogenesis, including the time required to traverse each of the stages, and the changes that occur in the type and number of cells throughout the stages. The events of cleavage and gastrulation occur within the first 48 h. Cleavage is holoblastic and unipolar and leads to a single-layered coeloblastula. Gastrulation occurs by ingression and is followed by the deposition of the thick cuticle. Thereafter, during the variable period of dormancy ranging from 2-24 weeks, little occurs; the important events are the conversion of the outer layer into an ectoderm and the appearance of the interstitial cell lineage. During the last 2 days before hatching, the endoderm and gastric cavity form, while stem cells of the interstitial cell lineage proliferate and differentiate into neurons, nematocytes, and secretory cells. Finally, the cuticle cracks, and the hatchling enlarges and emerges from the cuticle as a functional animal. The formation of the gastric cavity and the hatching of the embryo are both explicable in terms of the osmotic behavior of the animal and the hydrostatic forces generated by this behavior. Characteristics of development that are common to hydra and triploblastic phyla are presented.