Halliday (2008: 185):
The next of the three complementarities to develop is that between grammar and lexis. This is achieved when the child moves out of protolanguage into mother tongue, typically during the first half of the second year of life. In the protolanguage there is no stratum of lexicogrammar — this has not yet developed; so obviously there can be no grammar and no lexis. The protolanguage is not grounded in reference, so there are no resources for the referential mode of meaning. Referential meaning develops with the mother tongue, beginning with the move into “common” as opposed to “proper” names (that is, naming classes rather than individuals), and then opening up this new semiotic dimension as the meaning potential expands to accommodate both particularised and generalised meanings. — meaning as lexis, and meaning as grammar.