Halliday & Matthiessen (2014: 24-5):
In infants’ protolanguage, which has as yet no grammar in it, the elements are simple signs; for example, a meaning ‘give me that!’ is expressed directly by a sound, like nananana, or maybe by a gesture of some kind. Here we have just two strata, a stratum of content and a stratum of expression (cf. Halliday, 1975, 2004).
Adult languages are more complex. For one thing, they may have two alternative modes of expression, one of sounding (i.e. speech) and one of writing. More significantly, however, they have more strata in them.
The ‘content’ expands into two, a lexicogrammar and a semantics (cf. Halliday, 1984a; Halliday & Matthiessen, 1999). This is what allows the meaning potential of a language to expand, more or less indefinitely.