Halliday & Matthiessen (1999: 612-3):
But the immediate significance of the protolanguage is that by acting semiotically in these particular contexts children construe the fundamental distinction between "self" and "other", and the further distinction of "other" into persons and objects (cf. the discussion and figure in Halliday, 1978b). The consciousness of the self arises at the intersection of the various semiotic roles defined by each of these systems* — as well as, of course, from awareness of being one interactant in the general dialogic process (Halliday, 1991).
*Halliday & Matthiessen (1999: 613n):
There is a sense in which these rôles anticipate the functions in the transitivity structure of the clause: proto-Beneficiary (instrumental), proto-Agent (regulatory), proto-Carrier (interactional), and proto-Senser (personal).