Halliday & Matthiessen (1999: 613-4):
How, and why, do children discard their functioning protolanguage and move on to "language" in its adult form? To take up the "why?" first: because the protolanguage sets limits on both dimensions of meaning.
You can converse in it, but you cannot build up a dialogue: that is, it allows exchange of meaning, but it precludes any form of an interpersonal dynamic, in which meanings expand on the basis of what went before.
You can point with it, but you cannot refer: that is, it allows focus on an object, but it precludes any form of ideational systematic, in which phenomena are construed as configurations and in taxonomies.
For these to be possible you need a semiotic of a different kind, one that allows for a purely abstract level of representation "in between" the two faces of the sign, To put this another way (as we did at the beginning of the book), the sign has to be deconstructed so that, instead of content interfacing directly with expression, the relationship is mediated by a systematic organisation of form (a lexicogrammar). In other words, the semiotic has to become stratified.