The ultrastructural observations on the cocoon shell formation in Dugesia lugubris s.l. showed that, during the first developmental stages, it is formed by three different layers. Subsequently they become two and later only one, the inner, going to form the mature cocoon shell. We propose for the shell formation an activator coming from the genital antrum wall in addition to the substances produced by the yolk cells. This activator is supposed to be contained in the little vescicles located in the first and particularly in the second layer and opening themselves into the canaliculi of the third layer.