Among many aspects of ultrastructural studies on graptolites of erucial significance is the interpretation of their mode of secretion of periderm. This might contribute to solving the phylogenetic problems and to a be ter understanding of the development and the function of the colony. A membrane model of secretion of the graptolite periderm is discussed in some details and compared with alternative interpretations suggested by Anbres (1977) and Crowther and Rickards (1977). Both interpretations are evaluated inl respect to the entirety of the existing data. A generalized explanation of cortical bandages or ribbons formation due to the presence of the Iierithecal membrane is offered.