The coelenterates provide a simple system for the study of the processes of cellular differentiation and form regulation. The hydroids, such as the common hydra, are composed of only two layers of cells and a small number of easily distinguished different cell types (1, 2). The epidermis contains primarily epithelial-muscular cells, interstitial cells, and cnidoblasts. The epithelial-muscular cells contain contractile elements, myonemes, at their basal ends and secrete a coat of mucus at their apical ends (Fig. 6). Scattered between the epithelial-muscular cells are numerous small basophilic cells, the interstitial cells (Fig. 8). The interstitial cells occur in nests or small groups and normally differentiate into cnidoblasts (3). When the animals become sexual the interstitial cells give rise to the germ cells (1). The gastrodermis contains three basic cell types. The digestive-muscular cells are large cells which engulf partially digested food particles and which possess basal contractile elements similar to those found in the epithelial-muscular cells. The gland cells are small basophilic cells dispersed between the digestive cells (Fig. 7). They have a well developed endoplasmic reticulum (Fig. 9) and their apical ends are usually filled with secretory droplets (Fig. 7). The third gastrodermal cell type is the mucous cell. This resembles the gland cell but contains a mucous secretory product. The distribution of these cells varies considerably between different species. In Hydra viridis they are found only in the hypostome. The coelenterates are noted for an ex-