Cell movements in morphogenesis of hydroid polypes

1. Growth, morphogenesis and cell movements were studied in Obelia loveni, O. geniculata and Dynamena pumila with the use of time-lapse cinematography, visual observations of vitally stained objects and by histological techniques. 2. Growth pulsations with the period around 14min and the amplitude around 15 μm exist in Dynamena pumila and with the period 5–8 min and amplitude up to 5 μm in Obelia loveni. It was demonstrated that the rhythm of growth pulsations does not coincide with the rhythm of periodical contractions of the proximal part of coenosarc. 3. The distalwards movements of individual cells in the ectoderm of growing stems and hydranth rudiments are described. A considerable variability in the rates of movements of ectodermal cells has been demonstrated. 4. Different kinds of cell reorientations in developing rudiments are described. As a rule, they precede the alterations of growth directions or of rudiment shapes. 5. The mechanisms involved in deformations of epithelial layers are discussed. 6. The possibility of the existence of passive, elastico-plastic structures in the deforming epithelial sheets is suggested.