This paper presents a case study on the acquisition of grammatical morphemes via fillers, i.e., underspecified place holders, with particular focus on early structures made up of a filler followed by an infinitive. The path leading from fillers to French semi-auxiliaries and subject clitics is analyzed within the framework of Natural Morphology and constructivism which assumes that grammatical modules are not innate but are constructed by children. The evolution of fillers in the corpus studied is described as a grammaticization process of form and meaning through successive linguistic dissociations. Emphasis is put on the functional polyvalence of fillers and on their relation to the main phases in the construction of grammar.